Automotive Bodywork: The Secrets to Filling

I don’t expect to work in a body shop any time soon, but if that should come to pass, I would like to do collision metal work, bumping, and metal finishing damaged panels. If am ever employed that way, it will be one of my main objectives to give the paint-prep guys as little to do as possible. I have nothing personal against them, or their employment. It is just that good metal work should require little or no filling before it is coated with primer and paint. This is the last stage of metal work and, as far as the eye can see, probably the most important.


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However, like many other issues in metal work, the decisions of when, where, and how much filler to use are matters of degree. At some level, it is almost always possible to do enough metal finishing, without using fillers, to serve up panel work that needs nothing but primer and some light contour sanding to pass the muster of the highest standards. The question becomes, “What does it take to get the metal to that point?” At the very least, it means hours spent adjusting surfaces to perfection.


Looking from the panel’s back toward its front, you can see the completed repair of a decklid’s left-hinge mounting relief area. For demonstration purposes, the left side of the repair area was filled with body solder material and the right side was filled with plastic filler.

Looking from the panel’s back toward its front, you can see the completed repair of a decklid’s left-hinge mounting relief area. For demonstration purposes, the left side of the repair area was filled with body solder material and the right side was filled with plastic filler.


There comes a point of diminishing returns when you are making such adjustments, a point at which they may create other problems, sometimes elsewhere in a panel. That can mean backing up and doing creative destruction to your work, to try to eliminate that last niggling defect. That process may create other problems that have to be solved, often with great difficulty but without success.

Put simply, the issue becomes one of practicality, and of knowing when to quit and to use some filler to complete a job. Knowing when to quit is among the hardest disciplines to acquire in any line of work.

Body filler has a place. That place is as a corrective for minor surface defects, and for very small shortcomings in shape. If it is used within those limits, it is a full-fledged and respected member of the metal finishing family. If it is gobbed on to cover sizeable flaws in metal work, or used creatively to produce what amounts to sculpture, it is misuse.


The Secrets of Lead Work

An extensive example of lead work is included in Chapter 12, and in the photos and captions in the first part of this chapter.

Lead was the unquestioned filler of choice in body work from early automotive times into the early 1950s. This was true because it was the only known, practical body filler. Then polyester fillers were introduced and, over the years, have almost completely eclipsed lead fillers in this work. Polyester fillers enjoy many advantages over lead. By comparison, the material is much less expensive, and far less skill intensive and time consuming to apply. Unlike lead particles and fumes, exposure to airborne plastic filler particles is not particularly hazardous, though it is always a good idea to wear a protective filter mask when you sand plastic fillers. Modern polyester fillers are purported to be as durable as lead. They are also claimed to be at least as workable as lead when it comes to being filed and sanded to perfection.

I know that I am swimming upstream on this one, but I still prefer lead filler. Basically, I take issue with both of the last two points, stated above, that favor plastic fillers. While polyester fillers have evolved enormously in the last 50+ years, I continue to doubt that they have the durability of properly applied lead fillers. I also question that they can be finished as accurately. My reasons for doubting their comparable durability to lead is that plastic fillers remain somewhat water absorbent, even with the modern components that are now added as solids to their resins. They also lack the adhesion to base metal of properly tinned lead filler. Despite their improvement in these two areas, I think that they remain behind lead in both regards. However, that is only a personal opinion.

The earliest plastic fillers were resins filled with talc. Over the years, many other, less-moisture-absorbing substances, like marble spheres, have been combined with greatly improved polyester resins to make plastic fillers. The result is plastic fillers that greatly outperform the original issues of this type of product.

The bottom line for me is that tin/lead based fillers are metal. Applied to sheet steel, you have fillers that are somewhat similar to the base metal to which they are applied. Tin/lead fillers are far softer than the base metal, but they file, sand, and finish more like it than do plastic fillers.

You can decide for yourself which type of filler you favor in your work. I discuss both of them in the example that follows.

The decklid hinge mount area in the photograph at the beginning of this chapter shows damage that has been bumped back into its roughly correct shape, and then filled and metal finished, using both tin/lead body solder and plastic filler, in different areas of the repair. In the photograph, the tin/lead application is on the left side of the hinge-mount relief, while plastic filler was used on the right side of the relief. You can follow both processes, applied to similar situations, in the photos and captions that follow.


The Project

The panel used as an example here is the decklid from a Triumph TR-3, year not known. There was minor damage to the left-side hinge-mount area of this decklid panel.

Note: The convention of describing damage from the perspective of looking from the back of a vehicle forward should always be followed. Thus, in nautical terms, the damage described here was to the port-side decklid hinge-mount area.

The damage extended from the hinge mount’s center relief to the outer edge of the decklid. Put simply, the outer edge of the hood was sprung out in the hinge-mount area, and the hinge-mount area itself was canted down and to the left.


Before this repair was begun, the panel repair area looked like this. This photo was taken from the front of the panel. Note that the relief area was pushed down on the left, and the metal beyond that was sprung up and out from its edge.

Before this repair was begun, the panel repair area looked like this. This photo was taken from the front of the panel. Note that the relief area was pushed down on the left, and the metal beyond that was sprung up and out from its edge.


The nature of this damage, and how it was removed, is not central to this account of using body fillers, but a little information about it provides some useful background for that discussion.

The decklid arrived with its hinges removed. My guess is that it was caught in an open position by strong wind, or some other force, coming from the car’s right rear. This probably resulted in breaking the right hinge, while forcing the decklid panel up, and to the left. This would have bent and canted the left-hinge-mount area because the right hinge was no longer attached to the panel, and could not restrain its movement. The left-front lip of the decklid probably contacted and engaged the decklid jamb, springing the metal outward from the hinge-mount area. The visible damage to this panel was consistent with this scenario, but other sequences are possible.


Decklid Panel Repair

Step 1:

3After light abrasive blasting with silica sand, the damaged area looked like this. The damage is more obvious with the paint and rust removed from the panel. You can see a reinforcing plate through the front hinge-mounting hole in the hinge-mount area.


Step 2:

4A cardboard template was made from the undamaged side of the decklid. Then it was turned around and fitted over the damaged area. It indicates the exact location and extent of the panel deformation.


Step 3:

5Twisting the hinge-relief area sideways, up and away from the decklid’s edge, while lightly tapping the metal beyond it with a body hammer, returned the metal to its correct format. Two incremental applications of this procedure bumped this area back into proper shape.


Step 4:

6Filing the repair area began to reveal high and low spots. Body files are usually held with both hands, and slid forward and sideways, with a toe-to-heel weight shift as they are moved. Shown here is a one-hand filing motion, used to isolate a particular high spot.


Step 5:

7About halfway through the initial filing, the panel looked like this. Note the low spots along the right side and to the front of the hinge-mount relief area.


The deformation of the metal, immediately to the left of the hinge-mount area, was sufficient to release the paint there. Then, exposure to moisture caused that area to rust. After cleaning the damaged area with a very light blast of silica sand, a card-board template of the hinge-relief area was cut from the undamaged right side of the decklid panel. Then, it was turned around with respect to the car, and applied over the dam-aged area to determine the exact nature and extent of the damage.

The repair was accomplished simply, by fitting a large monkey wrench over the damaged hinge-relief area, with a small wooden pad under the relief and a large one on top of it. Then, the wrench was pulled sideways, away from the dam-age, while the metal beyond it, to the left, was tapped lightly with a medium-crown hammer. This combination of steady pressure and light impact almost completely restored the panel to its original format in one planned operation that was repeated twice. Correct contour was confirmed by checking the repaired area against the template that had been made from the undamaged hinge-mount area on the decklid’s other side. After the second operation, it fit perfectly.

The strength of the repaired panel was checked by attempting to physically twist it in various ways, while observing the repair area. No movement in the repair area was observed. There appeared to be no weakness in the metal, and with two correctly mounted hinges, it is doubtful that it will show any tendency to lose its shape in the repaired area. Since the hinge-mount area can be highly stressed when the trunk is opened, it is interesting to speculate whether the greater adhesion and strength of the lead part of the repair gives it better durability than the plastic part of the repair.

The repaired area was filed to indicate any low or high spots. Some low areas were found and partially corrected by picking them up, off-dolly, while using a shot bag for backing. At this point a few very shallow depressions in the metal remained, most prominently behind the hinge-mount relief area and to its immediate left. It might have been possible to raise these areas to level with more pick-hammer work, but their depth was so shallow that filling them seemed like a better approach.




Applying Lead Filler Material

The first step in applying lead filler is to clean the area to which it will be applied with a good, degreasing solvent. The adhesion of the tinning solder that bonds the lead filler to the panel is compromised by any traces of rust, paint, oil, or grease. Always degrease any area that you plan to fill with lead. This is done by wiping it down with rags, doused in solvent, and then wiping the area dry. This should be repeated until the solvent-wiping rags and drying rags come up clean.

Step 6:

8Before the repair area could be tinned, it was repeatedly wiped down and dried with enamel reducer. This removed any grease and oil from it, as well as any filing particles that remained on it.


Step 7:

9The metal in the repair area was now heated, in preparation for 50/50 tinning solder application. Low and high spots, left by the filing operation, are clearly visible. The low spots were shallow, and numerous enough to justify applying lead over the whole area.


Step 8:

10As the tinning flux was squirted onto and brushed around the repair area, it sizzled and steamed. This was a good indication that it was doing its job. The purpose of flux is to clean base metal and to promote solder flow.


Step 9:

11After the panel repair area had cooled a bit, excess flux was wiped off. It is important not to wipe all the flux off, or it cannot do its important job.


It is usually a good idea to lightly sand or blast the metal to be tinned with a fine-grit abrasive paper or blast media, to remove oxides and protective coatings. This should be done before it is solvent washed, so that the solvent washing removes sanding debris.

Although metal may look clean after sanding and solvent washing, it still has oxides and contaminants on its surface that interfere with the proper adhesion of the tinning solder that is used to bond lead filler material to it. To remove these adhesion-robbing contaminants, and to perform other wetting and adhesion-promoting goals, apply a flux to areas to be leaded. There are many types of liquid, powdered, and paste fluxes on the market. Some of them include solder particles in their formulations, and make fluxing and tinning a single-step process.

I prefer to use a straight liquid flux that is designated specifically for autobody lead work. This is available from many autobody and catalog sources. It is applied to metal that has been heated to the point that it sizzles off the liquid flux, when it is sprayed or dripped onto it. Flux application turns the metal a light gray color.

Step 10:

12Next, 50/50 solder was applied from a coil to the fluxed, heated metal. Note that the torch was not applied to the solder directly, but to the panel metal. You can see a completely tinned area behind where the solder is being applied.


Step 11:

13The solder was applied from the coil, and complete tinning was achieved by using a rag to rub it around on the panel’s heated area. You may accidentally singe a few rags as you are learning to do this.


Step 12:

14It takes some practice to stub lead from a bar onto tinned metal. The trick is to keep the tinned metal hot with the torch’s outer flame, while softening the end of a solder bar enough with its inner flame to twist it onto the panel.


Step 13:

15As the lead-shaping work began, the paddle was lubricated, to keep it from sticking to the lead as it was spread. To do this, we liquefied the surface of the tallow with the end of the torch flame and dipped the working surface of the paddle into it.


The best flame to use for fluxing, tinning, and applying body lead is one from an air-acetylene torch, often called a “plumber’s” torch. This type of torch produces a flame that is long, and graduated in temperature along its length. There are inexpensive air-draw tip attachments for oxy-acetylene torches that convert them to air-acetylene operation. These are a bit clumsy, but they do work. Note that an oxy-acetylene flame is much too hot for performing lead work. Propane torches are sometimes used for this work, but their flame is not as long and graduated as is the air-acetylene flame.

After the target area has been fluxed, it is a good idea to scrub off any excess flux residue with a rag and/or spun-nylon pad. These residues may appear as heavy, brown, gummy deposits, or they may not be present at all. Be careful not to scrub down to bare metal because a flux coating is necessary for adhesion of the tinning solder that you apply over it.

Tinning is accomplished by keeping the fill area hot with your torch, while melting enough 50/50 tin/lead solder from a coil onto the metal to cover the area when that solder is spread uniformly over it. It is important to use the middle-to-end part of your torch flame to keep the metal hot enough to melt the solder onto it, but not to overheat it by getting it much hotter than that.

Never apply your flame directly to the solder from the coil. As you melt solder from the coil onto the base metal, keep playing the torch over several square inches of the area that you are tinning. Then, when you have deposited enough solder to do the job, spread it out with a rag until it thinly and consistently covers the heated area. The solder should have a shiny appearance as it bonds to the metal. Be careful to wipe it lightly when you spread it, or you will remove the solder completely, rendering your tinning procedure useless.

After the entire area to which you plan to apply lead is tinned, it is time to apply the body lead. My preference is to use 1/2-pound bars of 30/70 body lead. Appendix I shows the transition temperatures for various tin/lead solder alloys, from solid to paste to liquid. My experience tells me that 30/70 is the best alloy for most body lead applications, with some minor exceptions, like sealing seams, which may be better served by using a 20/80 alloy. A 30/70 lead bar has a plastic state of about 130 degrees F. As long as you keep it in that temperature range, it will have a consistency something like peanut butter. In this state, it is easy to work with leading tools.

Step 14:

16We worked the lead with the end of the torch flame to heat it uniformly to its paste state. We did this slowly enough to heat through the entire filler thickness, not just its surface. Note that while the lead was being heated, the paddle was held at the ready.


Step 15:

17We repeatedly tested the lead surface with the paddle, to indicate when it reached a plastic state. A short time after it first become plastic, it was ready to work with the paddle.


Step 16:

18The paddled lead in this example looked like this when its application was completed. The dark residues on its surface are tallow lubricant and flux residues from tinning that worked up through the lead.


Step 17:

19The tallow and flux residues were removed before filing began. To do this, a metal conditioner was wiped onto, and scrubbed off, the leaded surface. This removed these contaminants.


If you overheat lead, it goes to liquid and, likely as not, ends up on your shoes and/or on the floor. At the very least, when body lead is over-heated to its liquid state, its tin and lead components tend to separate. Trying to re-soften separated body solder, after it has re-solidified, does no good because the tin and lead will no longer be alloyed. Without the tin/lead alloy, there is no plastic state in which this material can be formed with leading tools. Of course, if you allow your lead to cool too much, it becomes solid, and resists any attempts to shape it with paddles.

Before you try to form lead, you have to deposit it on the area where you are going to use it. You do this by heating but not overheating the tinned area of the base metal by playing the last few inches of your torch flame over it. As you are doing this, hold a lead bar against the panel in an area where you want to deposit lead filler, also keeping the end of the bar warm with the torch. Slowly attack the base of the lead bar with flame that is nearer to the hot inner cone of your torch flame, until it begins to soften. You will see and feel this happening. When the lead on the end of the bar (about 1 inch or less) is soft enough to deform easily, twist it off and onto the panel. Then, move to another spot and deposit another stub. Continue this stubbing process until you have deposited enough lead where you need it to accomplish the filling.

At first, it is likely that you will deposit more lead than you need. Later, with experience, you will learn to deposit the right amount of lead filler for your filling purposes.

The chief obstacle for novices is overheating their work areas, and liquefying the lead. This is a common problem with a simple, but not obvious, solution. Overheating is made worse because people tend to move their torches sideways when they sense that it is occurring. That simply results in overheating another area, one adjacent to the area that was the original problem. The answer is to move your torch in-and-out from your work, not to the side, to control heat. This approach helps you to maintain your 30/70 body solder in the 130-degrees-F range in which it is plastic. It takes some time to perfect this somewhat unnatural torch manipulation but, after a while, it should become quite natural.

The best lead-working tools are paddles made from seasoned hard maple, boiled in mutton tallow. Mutton tallow is also the preferred working lubricant for these tools. Other popular lubricants, like beeswax and chassis grease, cause all kinds of problems and should never be used. To apply fresh lubricant to a maple paddle, you play the end of your air-acetylene-torch flame over the surface of a tin of mutton tallow, until it melts to a depth of about 1/2 inch. Then, dip the work contact surface of the paddle into it. Let the excess tallow drip off the paddle, back into the tin. The paddle is now lubricated.

Step 18:

20A body file in a flexible holder is my favorite general filing tool, my first choice of weapon for small and large body-solder filing jobs. It is versatile and accurate. That setup was used here to file the leaded surface.


Step 19:

21As filing continued, areas of base metal appeared through the body solder. Correct filing technique ensured that the continuous contours being filed into the repair area were the correct ones. It is always a good idea to file across as much leaded area in one operation as possible.

Step 20:

22Panel edges should be filed with a very delicate touch. Light pressure, accurate file position, and constant monitoring of shape are essential when you file panel edges. Any inattention can result in damage to a panel.


To work, or form, the lead that you have stubbed onto the work area, you use your properly lubricated paddle to spread it onto the areas where you need it. Deposit the lead a bit thicker than the approximate thickness to which you will finish it. Keep the lead soft by playing the end of your flame over it. While you keep it in its plastic state, it is easy to form it with your paddle. Of course, all of this takes some practice, but, with the benefit of some experience, it is not difficult to master.

Some situations may require leading tools that are not readily available. For example, if you need to lead a 11⁄2-inch-round shape, it will be possible, but difficult to accomplish this work with a flat or convex lead-paddle working surface. The solution is to make your own tool for this purpose. It’s just too inefficient to try to do the job with an incorrectly shaped tool.

In the example above, I would take a stick of kiln-dried hard maple, say 2 x 3/4 inches, and form a radiused, chamfered, and tapered end on it, starting at about a 13⁄4-inch-round diameter. Then, I would boil the new tool’s working end, for an hour or so, in a tin can with about 2 inches of mutton tallow in it.

This step is often left out of the leading process and which, when omitted, results in endless later grief. It is critical to kill lead at this point; that is, right after it is applied to metal and paddled into preliminary shape. Killing, in this case, means removing flux and lubricant residues from your work. If left in and on the lead, such residues would later raise havoc with primer and paint adhesion.

Chemicals used to kill lead residues have ranged from ammonia, vinegar, and other household substances, to the metal conditioners used to prep metal for painting. A good metal conditioner is your best bet for neutralizing contaminants in worked lead. Phosphatizing conditioners are among the most effective chemicals in this class, and are readily available from body shop supply outfits. A good move at this point is to use a nylon scrubbing pad with your metal conditioner, or other killing agent, to scrub off any visible tallow residues on your paddled lead filler surface. Flux residues from tinning that may have worked up through the body solder may not be visible but if they are there, and if you file them into your lead, they will cause trouble later.

You will need to perform the killing process again, when your lead filler has been filed into final shape. That is a precaution to make sure that these residues are completely removed from your work. The best time to remove them is now, when you scrub and neutralize them out of the paddled lead, before you file it.

Lead filler is shaped with body files and hand-sanding devices. Never, never, try to shape lead with any power-sanding or power-grinding tool. Lead can be absorbed through the pores of human skin, and ingested in saliva. It is very deadly in airborne and small particulate form, which is the form that it attains when it is power sanded. If you work with lead, be careful to avoid its fumes and small particles that contain it. Be particularly careful to cover exposed skin, particularly arms, wrists, and hands. I also highly recommend wearing a particulate face filter. The symptoms of lead poisoning do not appear quickly, but the effects of lead poisoning are extremely debilitating, even fatal.

Step 21:

23While you can accomplish most filing jobs with a flat body file in a flexible holder, some jobs and areas require file shapes different from that. The side of the decklid hinge-mount relief required some filing with both round and square files.


Step 22:

24This specialty bull-nose body file was very helpful in getting the correct taper next to the hinge-mount relief. Unfortunately, files like this are no longer manufactured.


Step 23:

25After filing the repair area, it was sanded. The first sanding application was with 80-grit paper, backed by a hard-rubber pad. Note that the area to the right has not yet been filled.


The theory behind line filing and board sanding filler is that a series of somewhat random motions will average the surface that you are filing or sanding into the continuous contours that are the basis for flats, simple curves, and crowns. This works because lead and plastic fillers are softer than the metal to which they are applied, so that metal sets the overall shape that is filed or sanded over it. It does this by providing hard contact points that guide filing and sanding.

This theory places two requirements on the use of files and board sanders. First, they must be used in random patterns that vary slightly in position, direction, and application pressure with each stroke. Second, after rough filing is completed, that is, the basic shape that you are trying to achieve has been cut into the filler, you must be very careful to remove very little material with each successive, random stroke. If you follow this practice, you file continuous flats, simple curves, and crowns that are guided by the metal under the filler. As filler is filed off, and that metal appears, it guides your file into cutting correct contours. For this to work, the metal under the filler that guides your file has to be accurate before you attempt to file it.

Remember, filler is not meant to create the shape of what you file. It is designed to do only what its name implies, to fill low areas, and to help to reproduce exact and valid contours by allowing you to make very minor corrections to them. Once rough filing to remove excess filler is completed, fine filing creates final shapes and contours.




Specialty files are often helpful in dealing with complex shapes, particularly the transition areas between different crowns. The rules for using them are the same as those for flat files and files, like adjustable curve files, with simple end-to-end curvatures.

The proper filing motion with body files is described in Chapter 7. In summary, it is a forward motion, away from you, with a lesser side-sliding movement and a shift of weight from the front of the file to its back. This motion promotes desirable, continuous contours.

Once lead filler has been filed smooth, and very close to final shape, finish it with sanding procedures. Never attempt sanding with your bare fingers. If you do, you are more likely to disrupt desirable surface configurations and put inaccurate depressions into your work than you are to improve it. Always back up abrasive papers with some-thing like a rubber pad to give them some rigidity and some consistent pressure. Such pads vary from soft foam to pretty hard rubber. Sanding boards use fairly hard foam pads to directly back abrasive papers. This helps them to average contours, and to avoid gouging filler.

Step 24:

26Specialized sanding tools, like this one, aid in sanding fine detail into some panel shapes. A 180-grit paper was used to sand detail into the side of the hinge-mount relief area. Fine abrasives cut slowly, thus helping to avoid going too deep into lead filler.


Step 25:

27Major shapes and contours were fine tuned with a board sander fitted with 180-grit abrasive paper. The general contour of the panel was blended with the hinge-mount relief area that earlier had been detail sanded.


In some situations, you may need a specialized sanding tool to create a special shape or detail. This can be formed by anything from wrapping abrasive paper around a tapered file, to making a specialized wooden sanding tool to sand the detail that you seek.

After lead is filed, you can rough sand it with an 80-grit paper. This gives pretty fast cutting, while avoiding the creation of deep scratches. Finish the lead sanding with papers between 120- and 180-grit, depending on how far you want to go. Whatever sequence of filing and sanding you use, it is important that each stage of it removes any stray marks and scratches that are left over from the previous stage of your work. I always try to do a final board sanding of as much area as I can access with a long board sander. I do this after I have filed and sanded all of the details that I need.

As you file and sand filled surfaces, check them visually and by feeling them through a rag. One great advantage of lead filler is that as it is filed and sanded, it becomes shiny enough to reflect light reliably. This means that by looking at a con-tour and changing your sight line as you do so, you can watch reflections move across or along that contour. These indicate any defects in flats, curves, or crowns that may still exist. These reflections should move consistently, and without much change in basic distortion, until they encounter changes in crown.

For example, if the fluorescent tubes in an overhead light fixture are the reflection that you are tracking, they will be distorted by the curvatures of the panel from which you see them reflected. As you move your head with respect to that panel, the tubes will seem to move along it. The tubes will appear to be distorted, and the distortion in this reflection changes as you change the place from which you view it, and as the crown or curvature of the panel changes. But if you see a sudden change, particularly one in just part of the reflection, you may be looking at an undesired change in the panel’s contour that was a result of your metal finishing work to the panel or to the filler. Sudden changes should only occur in areas of fairly steep crown changes in the panel. Make sure that they occur only where they belong.

After a panel is finished in deep lacquer or glossy enamel, defects like the ones you can see in reflections stick out like warts on the heads of bald men. When you are evaluating your filler and metal finishing work, it is important to catch defects while they can still be corrected relatively easily.


Applying Plastic Fillers

A detailed example of a repair area that was filled with plastic filler is described in Chapter 13. What follows is a discussion of the theory of and best practices for using this type of material.

Probably, you already have the idea that I prefer lead filler to plastic filler. Despite my preference, the world has gone almost entirely to plastic fillers. Still, even if you never intend to use lead filler, you should read the first part of this chapter on that topic the one that you may have just skipped to get here because the shaping techniques and tools used for both types of filler are very similar in many particulars.

One great advantage of polyester fillers is that they are easier and faster to shape than lead fillers. On large jobs, they can be grated quickly into rough shape, before they fully harden. After they cure, they can be worked with power tools like disc sanders, DA orbital sanders, and air tools with grinding burrs.


Applying Plastic Fillers

Step 1:

28It is important to thoroughly mix the components of polyester filler separately, as well as together. Tubes of hardener should be kneaded, and cans of filler should be stirred with a screwdriver or putty knife, depending on their size.




Step 2:

30Mixed filler, deposited on a steel sheet, was spread with a putty knife onto the fill area. It is important to work filler into base metal, to ensure its adhesion to that metal.


Step 3:

31The filler was then shaped onto the repair area with a flexible plastic spreader. At this point, it was important to avoid creating air bubbles in the filler, and to work out any air that may have become trapped in it.


Step 4:

32The filler was then allowed to cure to a semi-hard state. The time needed for this to occur varies, largely depending on the proportions of hardener and filler that you mix. In this case, a timer was set for 15 minutes, which turned out to be just about right.


With proper metal preparation, plastic fillers have excellent adhesion. What they do not have is much ability to resist water molecules. After you complete work with plastic fillers, it is important to protect filled areas from moisture and humidity by coating them with waterproof primer, and/or primer and top coats that provide moisture-barrier protection. Those poor souls you see driving around with bare filler showing on their cars’ bodies are in for a world of hurt, particularly after they paint over that moisture-saturated plastic filler, and trap the basis for future corrosion under the paint.

Preparing metal properly for polyester filler involves having it as clean and grease-free as possible. All visible signs of paint, rust, and flying smut should be removed from the substrate metal. Its surface should be given good tooth to hold the filler. That tooth is the crags, crannies, nooks, and other interlocking shapes that lock filler to the metal surface. The adhesion capabilities of plastic fillers are purely mechanical, not chemical.

Abrasives in the 50- to 80-grit range provide good tooth. Below that range, you leave scratches that are problematic because they tend to promote sand scratch swelling in paint applied over them to the metal adjacent to your filler. Above 80-grit, metal lacks the tooth to adequately bond plastic filler. I have seen people who fanatically polished filler and metal with 400-grit, or greater, abrasives before applying primer to them. This is a bad idea because the primer will find little to grab onto in such a metal’s surface.

The first rule of using plastic fillers is to use a good-quality material. Some brands of filler are better than others. Reading product brochures and experimenting with different materials informs you on these issues. The second rule is to mix the components thoroughly, with themselves and with each other. When mixing them with each other the mixing proportions are not terribly critical, but stay within the range of proportions stated by the material’s manufacturer. This is sometimes expressed as the color of the mixed components.

Step 5:

33After the filler had reached its semi-hard state, it was grated with a cheese-grater-type file. This process can remove material very quickly, so you have to avoid over-enthusiastic grating at this stage.


Step 6:

34Plastic filler is filed in much the same way as lead filler. The same body files used for lead can be used with plastic fillers, with one bonus: There is less tendency to accidentally file lateral grooves into plastic filler.


Step 7:

35A round, tapered file, wrapped in 80-grit abrasive paper, was used to file detail into the hinge-mount relief area. Later, a tapered, square file, wrapped in abrasive paper, was used to further refine the shape of this area.


Step 8:

36As with lead filler, the mainstay of sanding this plastic filler was a board sander fitted with 80-grit paper. You can see how the board’s position has been shifted slightly with each stroke, so that the same stroke is never sanded more than once in the same location.


Unlike lead, polyester filler is inexpensive and comparatively fast to work into desired shapes. Also, unlike lead, polyester filler can easily be added on top of itself to raise low spots that may be revealed by filing. Adding it in three or four stages is common. With lead, such additions are difficult. It is a bad idea to use plastic filler to a depth of much more than 1/8 to 1/4 inch, and generally less is better. Do not allow the easy layering capability of plastic filler to encourage you to build up excessive thicknesses with it.

Polyester components should be rigorously mixed individually, to uniform consistencies, by kneading or stirring. Then, they should be mixed with each other on a hard and non-absorbent surface like sheet-metal or shiny plastic. They should not be mixed on surfaces like card-board or on any waxed surface, such as waxed cardboard. In the first case, the filler components can be selectively absorbed into the cardboard, unbalancing the chemistry of the filler. In the second case, the wax tends to mix with the filler and mess up its density and adhesion.

After the two components in polyester filler have been combined, they should be mixed thoroughly, and I mean thoroughly. Basically, you should mix them with your choice of mixing tool as completely as possible, and then mix them some more. When combined in the correct proportions, curing times are leisurely enough to avoid a problem with their setting up too fast.

Apply the fully mixed filler to the target area, and work it into its metal, again with your choice of tool. A flexible putty-knife blade or plastic spreader works well for this. After the filler has been applied, it should be spread with a plastic spreader and smoothed out into approximately the contours to which it will be filed. As filler is spread this way, it is important to not trap air bubbles in it, and to work out any air inclusions that may have occurred when it was applied. It should be left on the panel with a smooth, continuous appearance.

The curing of filler is a chemical reaction that will vary in speed with several factors. Among these are the ratio at which the filler’s components were mixed, the ambient temperature, and the thickness of the filler. Manufacturers of these materials supply rough data on set times.

When plastic filler reaches a semi-hard state, which can be determined by checking a sample of what you applied by impressing one of your fingernails or a tool into its surface, it is time to grate it. This is done with a cheese-grater-type file. The purpose of doing this is to save time later, by removing what is obviously excess filler more quickly and easily than would be possible with power or hand filing and sanding approaches. Be careful not to go too far in removing material at this stage, or you may have to add more filler later to make up a deficit. While this would not create any quality problems, it does waste time and effort.

Step 9:

37A shorter board sander was used to blend the detail from the hinge-mount relief into the adjoining panel filler. Note that the abrasive paper has been purposely positioned over the side of the sander, to let it ride up the relief.


Step 10:

38Some hand sanding with 80-grit paper, backed by a hard rubber pad, was applied to the back edge of the panel to give it shape. This was followed by sanding the area with 180-grit paper.


Step 11:

39The final step in this repair was to treat the bare metal in the repair area with metal conditioner. This is important because, without protection from airborne moisture, the bare metal could begin to rust in a few hours.


After the grated filler is fully cured (roughly, after lunch), it can be filed into exact contours, using the filing techniques discussed earlier. As noted, it also can be disc sanded to remove material and to bring it very close to final shape. The next adjustments and detailing of shapes should be performed with body files, using similar approaches to those used for this phase of this work for shaping lead. Again, as with lead, final shaping is done with abrasive papers backed with pads of varying hardness.

The final step in using plastic fillers is to treat exposed metal, adjacent to the filler, with a good metal conditioner. Unlike lead filler, plastic filler does not present the problem of flux and lubricant residues. Therefore, there is no need to kill or neutralize these contaminants. However, it is still a good idea to apply metal conditioner to exposed metal in areas where all filler has been filed or sanded completely off a panel. The best way to protect this metal, as well as to protect the metal under the filler from attack by moisture is to prime the entire panel or vehicle with a waterproof, etching-type primer, very soon after completion of final shaping and sanding. This provides the soundest possible surface for later coating with sandable primer and paint top coats.

Note: If you choose to use a metal conditioner, you should not use an etching primer over it. The two are incompatible. Use one, or the other. Over lead, it doesn’t really matter which you use. With plastic filler, the etching type of waterproof primer is the best choice because it deals with the lack of moisture resistance and moisture-absorbing potential of the plastic filler.

The photograph at the beginning of this chapter shows the completed repair of the decklid hinge-mount relief area. The area on the left, in the photograph, was filled and finished with body lead filler, while the area on the right was repaired with plastic filler. The panel is destined to be fully stripped and refinished for use on a car that is now in restoration. In the future, if this panel is ever again stripped for refinishing, someone will probably notice that half of the left-hinge-mount repair was made with lead filler, and that the other half was filled with polyester filler. I wonder what that someone will think.


Written by Matt Joseph and Posted with Permission of CarTechBooks


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