Automotive Bodywork: How to Clean, Model and Cut Sheetmetal

This chapter discusses the best ways to clean, model, and cut sheet-metal. These tasks are basic to all panel sectioning jobs, and to many fabrication projects.  They are crucial to everything that follows.

 


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Preparing and Cleaning Sheetmetal

Whether panel steel is old or new, it is important to clean it to bare metal at a fairly early stage of working with it. You can begin working with dirty steel; and sometimes it’s advantageous to start work with steel panels that still have old paint and/or rust on them. Filing and sanding these things off can help to indicate surface deformations and damage. In a sense, these defects can become a kind of guide coat as work progresses.

However, by the time you get to hammer forming or welding panel materials, you should be thinking about cleaning them. This is also true of new metal that is used in repair and for fabrication projects. While new steel-panel material looks clean, it is usually coated with preservative oil before it is shipped. This oil should be removed with an appropriate solvent before new metal is welded or passed on for painting.

 

Cleaning metal can be one of the most important and time-consuming preparations in metal work. If it is done properly, it should reveal the true condition of the sheetmetal, and indicate repair measures that may need to be pursued.

Cleaning metal can be one of the most important and time-consuming preparations in metal work. If it is done properly, it should reveal the true condition of the sheetmetal, and indicate repair measures that may need to be pursued.


 

There are many ways to clean sheetmetal. Body shop practice was once to treat it with muriatic acid (hydrochloric acid/HCL), or to use a blow torch to burn off old paint. Neither of these practices is recommended or even acceptable today.

Paint can be removed with any of several solvents designed for that purpose. These vary in speed and thoroughness. If you have to strip paint from a large panel, like a door or hood, general-purpose paint strippers will work, and aircraft-type paint remover is a particularly good way to do this job.

Disc sanding with abrasive paper mounted on a 7- or 9-inch electric sander is also an acceptable but slow way to remove paint. It has an advantage over chemical means of paint removal: It also removes surface rust. Old body shop practice was to use 36-grit, and even extremely coarse 24-grit, abrasive papers for this purpose. Open-coat abrasive papers in the 60- to 80-grit range are best for this purpose, and leave panel surfaces that do not require as much cleaning up to remove abrasive gouges.

There are many other varieties of spinning devices abrasive paper flap, wire, and mesh-pad-type wheels, for example that will clean metal. Some of these can work on 41⁄2-inch grinders, and produce very good results. Some of the plastic mesh discs sold for this purpose work surprisingly well for paint and surface rust removal. One great advantage that abrasive papers mounted on larger disc sanders (say, 7- and 9-inch devices) have is that they indicate surface defects and features, while removing paint and rust. Wire cup wheels are very good for digging into rust pits but tend to dull and bend quickly, losing much of their effectiveness.

If you are working with larger panels, like whole bodies, hoods, decklids, or doors, you should consider dip stripping as a method of paint and rust removal. At one time there were many dip-strip tanks in the United States. Unfortunately, some of the operators of these facilities disposed of their spent chemicals in irresponsible ways, like dumping them into city sewer systems. Since much of the paint dissolved in these effluents contained lead, many of these facilities ran into regulatory problems, and were forced out of business. There are still some dip strippers left, and, depending on your distance from one of them, this technique may be a good way for you to cleanse bodies and panels to bare metal.

My experience with dip stripping tells me that this service can vary greatly in quality and effectiveness between vendors. Some systems use heated chemicals. Many of them use electrolytic action to enhance cleaning. Some operators of these services are more careful about the chemistry in their tanks than others. In sum, if there is a reputable dip stripper relatively close to you, you should consider this method of cleaning your metal panels. In most cases, stripping a single door or decklid does not justify driving any great distance to a dip-strip tank. On the other hand, if you have a complete car to strip, or if you live near a dip tank, this method may be a valid approach.

 

Chemical paint stripper is one way to remove old paint. This is a general-purpose product. Aircraft stripper is a better bet for autobody panels. Muriatic acid and blow torches (on the right) were once used for paint removal. These were dangerous practices, and are now obsolete.

Chemical paint stripper is one way to remove old paint. This is a general-purpose product. Aircraft stripper is a better bet for autobody panels. Muriatic acid and blow torches (on the right) were once used for paint removal. These were dangerous practices, and are now obsolete.

 

Disc sanding works well to remove paint and rust from body panels. You have to be careful with this technique not to overheat metal and warp it. Done properly, disc sanding is a slow metal-cleaning method.

Disc sanding works well to remove paint and rust from body panels. You have to be careful with this technique not to overheat metal and warp it. Done properly, disc sanding is a slow metal-cleaning method.

 

Twisted wire cup brushes, like this one, are good for digging into rust pits in sheetmetal. It is important to change them often because their wires bend and their ends dull quickly.

Twisted wire cup brushes, like this one, are good for digging into rust pits in sheetmetal. It is important to change them often because their wires bend and their ends dull quickly.

 

With considerable patience, abrasive blasting works well to clean sheet metal thoroughly. You have to be careful to work slowly, or you will warp the metal. Media-rich

With considerable patience, abrasive blasting works well to clean sheet metal thoroughly. You have to be careful to work slowly, or you will warp the metal. Media-rich air/abrasive mixtures, low numerical blast angles, low blast pressures, and long nozzle-to-target distances help to prevent distortion.


 

Abrasive blasting, sometimes called sandblasting, is another major category of metal-cleaning technology. Blasting is a terrific method for cleaning metal if it is performed by a skilled professional. In addition to getting metal metallurgically clean (as the abrasive blasting industry claims for some blast media), this approach leaves an excellent tooth on finished metal. This means that the surface will have nooks, crags, and crannies that will help paint to mechanically adhere to it.

 

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There are two downsides to abrasive blasting to clean autobody panel metal. The first is that blasting tends to expand surfaces to which it is applied. Media, like glass bead, do this in the extreme. When you expand one side of a surface, but not the other, the metal tends to bend around the unexpanded side. Put simply, it will warp. And, no, you cannot blast the other side to bring it back into proper shape. I wish that it was that easy.

I have seen fenders and doors ruined beyond the possibility of repair by what someone thought was safe blasting. The only way to safely blast sheetmetal is to do it very slowly. Soft or small blast media, like soda and polystyrene beads, are very slow ways to clean metal. They do not remove deep, pitted rust very well, if at all, but they do avoid the warping hazard. Harder media, like silica sand, tungsten carbide, aluminum oxide, and cold chilled copper slag, will produce very clean metal. But they must be used slowly, due to their potential to warp metal. This means a combination of a numerically low nozzle-to-target angle, low blast pressures, media-rich mixtures with air, and/or long nozzle-to-target distances.

Any of these approaches, or combinations of them, makes the blast removal of paint and rust from sheetmetal very slow. However, if you are willing to exercise the restraint to do this, which is often very difficult, you can blast sheet-metal clean with good results.

The second downside of blast-cleaning metal is that it leaves it so clean that any moisture in the air tends to rust it almost immediately. In dry environments this is a smaller problem than in wet ones. In all situations, blasted parts should be worked on quickly, re-cleaned, and then painted or treated with a preservative immediately.

 

Cutting Panel Materials

There is no perfect, or best, way to cut sheetmetal materials. Different methods of cutting metal work best in different situations.

If a straight line is required, it is best to shear flat metal in a metal shear. A stationary or hand shear is a very good approach to delicate detail work. Another possibility is using pneumatic and electric shears of various types. Tin snips and aircraft-type snips are good for small shearing jobs in flat and curved metal. Band sawing is a very good way to cut flat metal stock very accurately and quickly. Unfortunately, good metal band saws are quite expensive.

Reciprocating pneumatic saws offer a very effective and inexpensive way to cut detailed lines in panels, and are particularly useful for cutting bad metal out of large, contoured panels. High-speed abrasive discs mounted in air grinders are another way of making accurate cuts. This method is favored by many body shops and custom builders. Electric hand shears are also very accurate and fast for making simple cuts.

 

If you are lucky enough to have a good jump shear like this one, you can make straight, clean cuts in sheetmetal almost effortlessly. Hand-cutting techniques for this kind of job are never quite as good.

If you are lucky enough to have a good jump shear like this one, you can make straight, clean cuts in sheetmetal almost effortlessly. Hand-cutting techniques for this kind of job are never quite as good.

 

Band sawing is a great way to make accurate, smooth-cut lines in flat sheetmetal. Unfortunately, a good, metal-cutting band saw is an expensive piece of equipment.

Band sawing is a great way to make accurate, smooth-cut lines in flat sheetmetal. Unfortunately, a good, metal-cutting band saw is an expensive piece of equipment.

 

A stationary hand shear, like this Beverly shear, is a great way to cut very accurate shapes into flat sheetmetal. This type of shear is throatless, so there is no limit to the depth of cut that you can make with it into panel steel.

A stationary hand shear, like this Beverly shear, is a great way to cut very accurate shapes into flat sheetmetal. This type of shear is throatless, so there is no limit to the depth of cut that you can make with it into panel steel.

 

Aircraft snips are great for making accurate cuts in flat sheetmetal, and work well on mildly curved surfaces. They cut reasonably quickly and are relatively inexpensive.

Aircraft snips are great for making accurate cuts in flat sheetmetal, and work well on mildly curved surfaces. They cut reasonably quickly and are relatively inexpensive.

 

Air-driven saber saws are cheap and are capable of making accurate cuts in contoured sheetmetal. They are the body shop standby for excising diseased areas in panels.

Air-driven saber saws are cheap and are capable of making accurate cuts in contoured sheetmetal. They are the body shop standby for excising diseased areas in panels.

 

High-speed abrasive disks, mounted in air die grinders or muffler cutters, are great for cutting into contoured panels, but are pretty much limited to cutting straight lines.

High-speed abrasive disks, mounted in air die grinders or muffler cutters, are great for cutting into contoured panels, but are pretty much limited to cutting straight lines.

 

This type of electrically driven hand shear cuts metal quickly and accurately, and is not limited to flat surfaces. Air-driven versions of this tool lack sufficient torque, and do not work as well as electrically driven versions.

This type of electrically driven hand shear cuts metal quickly and accurately, and is not limited to flat surfaces. Air-driven versions of this tool lack sufficient torque, and do not work as well as electrically driven versions.

 

Carbide burrs mounted in air die grinders, like the angle grinder shown here, are a terrific way to fine tune metal edges for fit-up to other edges. These tools are surprisingly inexpensive.

Carbide burrs mounted in air die grinders, like the angle grinder shown here, are a terrific way to fine tune metal edges for fit-up to other edges. These tools are surprisingly inexpensive.

 

Plasma arc cutters cut very quickly, but lack the accuracy of mechanical methods of cutting metal. They work well on curved surfaces, and leave weldable edges. Despite the intense heat that they generate, they cause little distortion in metal because their cutting heat is contained very locally.

Plasma arc cutters cut very quickly, but lack the accuracy of mechanical methods of cutting metal. They work well on curved surfaces, and leave weldable edges. Despite the intense heat that they generate, they cause little distortion in metal because their cutting heat is contained very locally.


 

Nibblers, both air and electric, have some application to cutting autobody metal, particularly when there is no direct entry to begin a cut. Unfortunately, they are messy to use because they spew crescent-shaped metal fragments from their cut paths, all over creation. Find some use for those fragments, and you may become wealthy and appreciated by many. Hole saws and punches are also options for opening up areas, so that you can use other cutting methods when interior cuts in metal panels are required.

 

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Plasma arc cutting is a high-tech method of severing sheetmetal. It is very fast, and leaves edges that are still weldable, unlike cutting with an oxy-acetylene torch. However, the lack of fine control of plasma-arc cut lines restricts the use of this method to situations where fine accuracy along cut lines is not required.

Fine adjustments to metal edges are often made with die grinders and files. This is particularly true when parts are being fine-tuned for tack welding and welding. High-speed electric and pneumatic die grinders are also very useful for making these kinds of fine adjustments.

 

Getting Shapes and Contours Right

There are many ways to create three-dimensional models and forms for sheetmetal objects. These may vary from those that just indicate dimensions to be followed, to those that can be used, almost like dies, in the actual forming process. In either case, these models and forms serve as standards against which to check the fidelity of the metal that you are forming. Sometimes, the metal is intended to replicate some original part, section, or feature of a vehicle. Other times, it is designed to guide the construction of a custom part or panel for an entirely new fabrication.

Very advanced prototype shops have computer-driven systems and hardware that can sense existing or conceptual shapes, turn them into numerical coordinates, and automatically form them in various plastic materials or shave them out of blocks of clay. This kind of capability is wonderful to behold, but presently it is way beyond the scope or needs of most sheetmetal projects that don’t involve automobile prototyping and manufacturing.

 

Good, old metal files are great for straightening cut lines in metal edges, and for removing burrs and other imperfections. They can be used in combination with handheld edge deburrers to produce very clean edges.

Good, old metal files are great for straightening cut lines in metal edges, and for removing burrs and other imperfections. They can be used in combination with handheld edge deburrers to produce very clean edges.

 

These wire patterns almost look like art, but are really forms that are incredibly useful for checking metal as it is formed into faithful replicas of original parts. They are also quite durable.

These wire patterns almost look like art, but are really forms that are incredibly useful for checking metal as it is formed into faithful replicas of original parts. They are also quite durable.

 

Paper templates can be made as drawings or as three-dimensional representations of original metal. This paper model is a combination of drawing and contouring to capture the shape of a part that will be formed in metal.

Paper templates can be made as drawings or as three-dimensional representations of original metal. This paper model is a combination of drawing and contouring to capture the shape of a part that will be formed in metal.


 

My favorite method of transfer-ring most simple contours and dimensions from existing shapes to models is to use number-12 copper electrical wire, or grids made from this wire. Examples of this technique are shown in Chapter 3 and Chapter 12. Every shop has its own favored methods of modeling shapes. Wire forms are a favorite with many. Others have developed various modeling methods in paper, both with flat drawings and by forming the paper into three dimensions. Clay and wooden-buck modeling are steps beyond that.

When I want to model all of a shape in three dimensions, I usually use the old egg crate modeling construction. It is relatively quick, easy to do, accurate, and inexpensive in material costs.

 

Written by Matt Joseph and Posted with Permission of CarTechBooks

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