Preparation (“prep work,” or just “prep”) is certainly very important to a good, long-lasting paint job, but not necessarily in the same ways it used to be.
In the old days stripping the car to bare metal wasn’t nearly as prevalent, and we didn’t have high-fill, catalyzed primers. So carefully hand-sanding the entire surface of the existing paint was absolutely necessary to ensure a smooth, unblemished finish for the final paint layer. You had to painstakingly hand-sand out every rock chip, scratch, or crack, featheredging it until you could no longer feel any imperfection in the surface. A few coats of lacquer primer, in between sanding, helped. But the heavily solvent-based lacquer primer often seeped into exposed undercoat layers, especially in feather-edged areas, causing them to wrinkle or lift. Or, lacquer primers or putties would absorb solvents from paint layers, causing them to swell, or shrink, or both.
This Tech Tip is From the Full Book, HOW TO PAINT YOUR CAR ON A BUDGET. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: http://www.musclecardiy.com/bodywork/automotive-painting-guide-prep-sand-and-mask/
So painting a car in the fairly recent old days meant starting by laboriously hand wet-sanding the entire body (after any necessary bodywork, of course) with 180 to 220 grit paper, feathering out chips and scratches as mentioned, and then going back over the whole thing with 360 to 400 grit wet-sanding. The latter process might be repeated two or three times, with layers of lacquer primer in between. Painting with lacquers could produce glorious results, but the preparation process was laborious and could be very frustrating.
Preparation these days means something different and more inclusive. Today’s products are better, easier to use, faster, and certainly longer lasting.
Instead of starting the prep process with sanding the old paint, let’s rethink the whole procedure. Obviously, if you’re going to strip the car to bare metal (or fiberglass), that negates sanding the old paint entirely. Similarly, which of the following prep steps you follow depends more or less or the “stage” of paint job you’re doing on your vehicle. But the basics of prep include: (1) Straightening the body. This includes major and minor metalwork, use of fillers, and board-sanding surfaces smooth with 36 and 80 grit paper. (2) Cleaning and adhesion. We’ve covered products and methods for cleaning the surface, but before you sand anything, the surface must be clean of any dirt, oil, grease, or other contaminants such as silicones. And it must be re-cleaned more than once before you shoot final paint. Equally important, you must use paint products (undercoats or topcoats) that ensure maximum adhesion to whatever surface you’re dealing with, above or below it. A major, though often over-looked, part of the prep process is making sure the final paint does not bubble or lift anytime after it is applied. (3) Priming and sanding. If you’re repainting a relatively new car with decent paint, then maybe all you need to do is clean and fully sand the existing surface, possibly spray an adhesion coat, then shoot your color. But most cars that need a paint job need more than that. The details are discussed later in the book, but this is where high-fill primer (usually) comes in, and lots of sanding—block sanding, careful sanding, complete sanding in all nooks and corners. It’s still somewhat laborious, but usually not frustrating.
Let’s briefly delineate the basic steps of paint preparation in the order in which they should be done.
Disassembly: Start by detaching, or peeling off, anything that gets in the way of painting the basic surface of the car—that is, all that was originally painted by the factory (or more, if you want). This might even include removing some pieces (such as large, plastic bumpers or body “cladding” on newer cars) that should be painted separately and rein-stalled on or over the repainted body.
This step depends on which class of paint job you’re doing and your own levels of energy, patience, and perfection. When taking parts off, it’s a good idea to bag and label them and their fasteners, because it may be a while before you’re ready to reinstall them.
Bodywork. We won’t discuss this here because we covered it in Chapter 3, but it is definitely one of the steps of paint prep, and it should be done now, after disassembly (and paint stripping, if that’s included). If you have bodywork done by someone else, they usually cover the area with a coat of high-fill primer (unfortunately, many times so you can’t see what exactly they’ve done). The running jibe between bodymen and painters is that the bodyman gets the surface “close enough,” shoots it with sanding primer, and leaves it to the painter to block-sand smooth. Painters, of course, yell that the bodyman didn’t get it close enough. If you’re doing both bodywork and paint, you want to get it “really close” during the bodywork stage. Then, depending on other factors, you can shoot a coat of primer over the body-worked areas as you do them, or wait and prime the whole car at once, before block sanding.
Cleaning/Degreasing. This step might start with soap and water or a pressure washer. We’ve discussed degreasing agents. They come with directions, and should be used before any new layer of material is sprayed on the car if you’ve worked, touched, or sweated on it in between. If you strip the car to bare metal, you can skip this step for now.
Sanding. If the paint on your project vehicle is significantly cracked, checked, peeling, bubbling, or flaking, you are much better off stripping it down to bare metal (by whatever method you choose). If the current paint on the car is not adhering properly, whatever paint you put on top of it will not adhere properly either.
Stripping paint “by whatever method you choose” includes those we have discussed—it does not include sanding all the paint off the car. If you try hand-sanding your car to bare metal you’re nuts or a masochist (probably both), and the same goes for (usually drill-motor-powered) strappers, flappers, rotary wire brushes, or whatever else is being hawked on late-night TV. There are good types, such as 3M abrasive wheels, that work well in small spots, especially for metal cleaning, but not for a whole car body. Most importantly, do not try to sand all the paint off your car with a rotary sander of any type. Most true metalmen cringe at the thought of a body grinder touching sheetmetal. Others, including plastic filler manufacturers, actually recommend using a coarse-disc grinder (i.e., 24 to 36 grit) to both clean and rough-up a body-worked area for best filler adhesion. I’ll go with that, to a minor degree. But don’t take a body grinder to the whole outside of your car—or even major portions of it—just to strip paint off. Just as bad (and I’ve seen this way too many times) is using any sort of rotary sanding discs, whether in your electric drill or its own machine, to sand down a car. These are for sanding the flaking paint on the wood trim of your house. On a car they only gouge the surface and grind ruts in the metal. At this stage of the paint job we assume any major bodywork has been done, and sanded as level and smooth as possible with at least 36-grit paper on sanding boards or blocks. We want to make the entire surface smoother, not gash and gouge it.
You don’t need air tools for the occasional at-home automotive paint job, at least not for sanding. But if you’re a tool collector, a couple types are acceptable. They are of the orbital (i.e, vibratory, rather than rotary) type, and they incorporate a semi-hard, flat backing to act as a block to help level the surface. Jitter-bugs come in various sizes, from hand-block up to Bondo-board. They are most effective when used with coarser papers to level out layers of plastic filler, but can be used with 180-grit dry paper to sand and level paint surfaces, too. Although a dual-action (D.A.) sander uses a round pad, it vibrates rather than spins. A favorite of production bodymen, it takes more practice to get surfaces smooth. But for the home-painter it’s good for feathering-out rock chips, deep scratches, and other surface imperfections in small areas. It’s not designed to sand large surfaces smooth.
Completely hand-sanding a car for paint, properly, only takes a good day or two, at most. When painting one car, at home on your own time, that’s not much. So take the time to do it thoroughly. Here are a few tips to make the job go well:
- Use quality sandpaper made for automotive paint work (3M, Makita, Norton are some common brands; there are others). I suggest you get it at your automotive paint supply store. It may seem expensive at first, but it pays for itself in the long run by lasting longer and not clogging as easily as general-purpose sandpaper. For hand-sanding, tear a full sheet of paper in half, then fold it in thirds. If you do enough sanding, buying sandpaper by the pack or box saves bucks, and saves trips to the store (especially late at night and on weekends when they’re closed, but you’re working on your hobby project).
- Use a semi-hard rubber pad inside the folded paper as often as possible to even the sanding pressure and give a blocking effect. Use your fingertips, and the corners of the paper, to get into corners and concave ridges, which are very important to sand. But—as you have undoubtedly heard many times—don’t sand with your fingertips in open areas. It leaves uneven ridges. Use the palm of your hand, or turn your hand at an angle. Also, especially with a block, do not sand parallel to an edge or a ridge. Sand at an angle. Otherwise the outer edge of the block/paper cuts a ridge in the surface.
- Speaking of fingertips—this one hint is worth the price of this book. The reason painters fold their paper in thirds is so it won’t wrinkle or ball up, is easier to hold onto, and can be turned regularly for a fresh surface (plus, it makes a good size to fit your hand). But as one side is sanding the car, the other side is sanding your hand. After a day of this, believe me, your hand is going to be bloody and it hurts! Especially your fingertips. I’ve seen painters wrap their fingers in tape to guard against this. But the hot tip is to wear a pair of rubber dish-washing gloves. You want the kind you can pick up a dime with; they come in large, and they stretch to fit your big hands. They’re especially helpful when wet sanding. In fact, get two or three pairs, because you’ll wear holes in them—instead of your fingers. You’ll thank me for this one!
Wet or Dry?
It’s your preference. Only “wet-or-dry” paper, which is typically black, works wet. You can use it dry—some do—but I don’t. The primary advantage to wet sanding is that the paper lasts longer and tends not to clog as much. It is messy, though, both on the car and on the floor; you have to wash the residue off, and you have to wait till the washed surface dries to see how you’ve done and whether it needs more sanding. On the other hand, dry sanding produces lots of dust, which some people hate more.
Personally, I wet-sand existing paint, starting with 220 to 240 grit and going finer. (Color sanding, which we get to, is always done wet.) Anything that requires 180-grit sanding, especially with a board or block, I prefer to do dry. And because of years of using lacquer primers, which can absorb and hold water, even when they look dry, I refuse to wet-sand primers of any kind. With modern catalyzed primers it’s really okay, and a lot of painters prefer to wet-sand everything. But I like the gray Fre-Cut paper for dry sanding; I think dust blows off a lot easier than cleaning up after wet sanding; and I’d much prefer to block- or board-sand with dry paper. After any kind of wet sanding, especially on primer, be sure that you have completely washed off all residue, and allowed the surface to thoroughly dry, before spraying any paint product on top of it.
You probably think you’re sanding the surface to make it smooth. That’s only partially true, and not nearly as true as it once was. You’re sanding for two reasons: (1) to level the surface, and (2) to scuff it up to promote adhesion of the next layer of paint. In the days of lacquers, which were thin and contained solvents that bit into the layer beneath it, it was common to do multiple layers of paint, sanding with 400- to 600-grit paper in between, to get a glass-smooth finish. Today’s catalyzed paints (especially with a clear coat) are thicker, and they “cross-link,” which means their molecules chemically bond with each other—but not necessarily with the layer below. So fine-grit sanding is not only a waste of time and effort (because the paint—or high-fill primer—fills in scratches and other imperfections), but can actually decrease adhesion of the next layer of paint. Most of today’s painters say that sanding to 240-, 320-, or 360-grit, at most, is plenty. If you’re sanding down an existing paint job, it’s much more important to make the entire surface uniformly dull and abraded than it is to make it silky smooth. This is especially true in areas such as doorjambs (if you’re painting them), around windows, in body creases and indentations, and so on.
Prime and Block. (Read Masking below, too, before you prime.) In the above scenario, we’re assuming the car has decent (undamaged, non-peeling) paint on it, and requires little or no bodywork. But most cars that need painting don’t fit that description. For these, we welcome catalyzed high-fill primers and catalyzed spot-putties. Yes, they can be abused. But, boy, do they make paint-prep easier than it used to be.
We don’t need to say it again—if the old paint isn’t adhering to the body, strip it. Once it’s stripped, do any necessary bodywork, board-sanding filler at least to 36 to 40-grit, if not 80-grit. Since you’re painting the whole car, not just doing spot bodywork, we then suggest you spray the whole body with a coat of sealer/etching-type primer, followed by a coat of high-fill catalyzed primer. If the body is really straight enough that it doesn’t need block sanding, fine; shoot it with the sealer and shoot your color coat. But I haven’t seen many bodies needing paint that are that straight. Most have some door-dings, scrapes, or other little wobblies that don’t require filler, but can be block-sanded out of a high-fill primer coat, or filled with catalyzed spot putty over the primer (the putty is not made to go on bare metal), and then block-sanded level. You could just spray the high-fill primer where you see it’s needed, but most painters find it faster and simpler in the long run to prime the whole car at once. The problem is that you usually can’t see (or even feel, with a well-trained hand) all the little imperfections that show up in the final, glossy paint coat.
The same usually applies to a vehicle you don’t need to strip. If it’s been around the block a few times, you’ve probably already done some bodywork to it, and it very likely has its share of rock chips, door dings, scrapes, and so on. First, wet or dry sand the existing paint in the manner, and for the reasons, out-lined above. However, you don’t need to fully feather edge every rock chip or scrape until it’s smooth. Just knock down the rough edges and make sure the paint isn’t flaking around the chip (check it with your fingernail—or something similar if you’re wearing rubber gloves). If the paint has any cracks or splits in it, these you have to “chase”—keep sanding them, even to bare metal, until you’re confident the paint remaining around them is adhering properly.
Then you have two choices. You can spot-paint high-fill primer over any body-worked areas, and all the chips, scrapes, dings, and minor dents that didn’t require filler, but might need some spot putty. Or you can spray the whole body with a sealer, followed by a generous coat of high-fill primer. This would usually be followed by a guide coat and block sanding of the whole body.
It all depends, once again, on how much work you want to do and how good a paint job you want.
The two things that separate a regular paint job from a first-class, show-quality one are full, careful block sanding of the entire body and equally careful color sanding and rubbing-out of the final gloss coat. Both add considerable time and work (though very little cost) to your paint job. It’s your time and your effort.
Block sanding has levels, or stages, just as paint jobs have levels. Since the majority of paint jobs don’t involve bare-metal stripping, the most common scenario is: do necessary bodywork; sand the rest of the car (as described); spot-in the body worked areas as well as any chipped, scratched, or dinged areas with high-fill primer; use catalyzed spot putty, sparingly, to further fill chips, scratches, dings, and very minor ripples or dents; block-sand these areas smooth with 80-grit paper (using a long-board on larger areas); and then respray these areas—and probably the rest of the car—with another coat of high-fill primer, followed by another round of block-sanding.
How many times you’re willing to do this, over what percentage of the vehicle’s body, and with what grits of sandpaper, determine what level of block sanding you achieve. It’s up to you.
Personally, after priming, puttying, and board-sanding bodyworked areas to 80-grit (dry), I prefer to prime the whole car (with high-fill, catalyzed primer) and block-sand the whole thing with 80-grit on a long-board wherever possible, using smaller blocks as necessary in smaller areas. By dry sanding with relatively coarse paper, I’ve never found the need for a guide coat. You know high spots when you uncover bare metal. Low spots are visible because they remain unsanded (sort of shiny). Depending on the magnitude of these high and low spots, they might require a little more tapping with a body hammer at this point (I prefer not to use putty in large low spots, only in small nicks or dings. Never add filler over primer; if an area needs further filling—and this is often discovered during the blocking stage—strip or grind it to bare metal first). Then spray more primer and continue block/board sanding until all high and low areas are gone. I then follow with a full-body sanding with 180-grit, dry, by hand, using a rubber pad, until all visible 80-grit sand scratches disappear (adding more primer, as needed, if I hit bare metal in spots). Then I do it again with 320 to 360-grit paper, dry, with or without a pad.
Those who prefer to wet sand while blocking benefit from a guide coat, because you can’t see low areas otherwise. The guide coat can be a light fogging of any different color paint, usually from a black spray can over a light primer. The type of paint doesn’t matter, because it all gets sanded off, if you do your blocking properly.
I should mention here that there is some controversy between painters and bodymen today over whether the majority of block sanding should be done with filler over bare metal, or with high-fill primer. I think it’s a moot point (in fact, there are a few brands of high-fill primer now that are essentially spray-on filler). Too much of either is bad, and not an acceptable substitute for proper metal-work and body straightening. But that takes more talent and experience. My personal opinion is that 80 to 90 percent of whatever type of body filler you put on the car should end up as dust on the floor.
Masking. I hate masking. It’s worse than wrapping Christmas presents. I’d prefer to remove parts than mask them, any day. And that is the preferred method for car painting, anyway. But some masking is usually necessary (especially for home hobby-type paint jobs, as opposed to body-off-frame restorations), and some rules and several tips apply.
We could have covered masking before priming/blocking, because you have to mask before you can start spraying anything, including primer. On the other hand, you need to remove and re-mask everything after block sanding, anyway, as you will see. Once again, some basics:
- The quality of masking tape varies greatly in terms of sticking, unsticking, bending around corners, and so on. Do not use cheap tape. I use only 3M brand, and the tan type (not blue or red, or what-ever). You probably need to get it at the automotive paint store. It’s not cheap, but cheap tape isn’t worth the price. Any amount you could possibly save with cheap tape is overshadowed by the annoy-ance and wasted time you suffer trying to apply and remove it properly.
- Likewise, use only automotive masking paper; it’s usually light green and comes in large rolls of various widths, again, at the auto paint place. Newspaper is free, but it’s cumbersome to unfold and fold, it is porous, it sheds lint, it holds dust, and the newsprint can rub off. Newspaper is not masking paper.
- They make caddies that hold a couple sizes of paper rolls, plus tape, that apply the tape to the edge of the paper, allowing you to tear off paper the length you want, taped and ready to go. They work great (when they work properly). But I’ve never gotten one, mainly because it takes up too much garage space for the little time I’d actually use it. However, the real tip is to attach the tape to the edge of the paper before you try to tape the paper to the car. Masking is tedious, but this makes it many times easier.
- Never leave tape on the car after it gets wet. Don’t mask a car and leave it outside over night. Once it gets wet and dries on the car, it might as well be glued on. It won’t peel off. And if it’s taped over paint, it can peel the paint off. If you’re going to wet-sand existing paint on the car, do it before you do any masking (assuming the windows are still in place). Wash the car off, and thoroughly dry it, before masking. If you’re wet-sanding primer coats, you must remove masking tape and paper before it dries on the car.
- Whether you wet-sand or dry-sand the car, always remove and replace all masking paper before shooting final coats. Otherwise sanding residue that collects in the folds of the paper (and tape) invariably blows out into your fresh paint as you’re spraying, no matter how hard you try to clean it out beforehand.
Other considerations regarding masking pertain to the level of paint job/car build you’re doing. Obviously, if you’re doing the bare body off the frame, you don’t have to mask anything. At the other extreme, if you’re doing the “scuff and squirt,” with the doors, hood, etc., closed, take off as much exterior trim as you can, and then mask all windows and everything else on the outside of the car you don’t want painted. In between are several variations, ranging from pulling all the interior and glass out, to masking (sealing) off these areas completely (if you leave the interior in). Bottom line, masking is pretty intuitive. If you don’t want paint on it, take it off or mask it off.
A couple of further things: In the old days, painters masked off the interior and shot the doorjambs first (and under the hood and trunk, if they were going to do them), in fast-drying lacquer, then closed them a few minutes later and shot the whole outside of the car in enamel (both mixed to the same color). These days, with relatively fast-drying catalyzed paints, most painters mask off the interior, windows (and engine, maybe), if they’re still in place, and paint the car, doorjambs, under the hood, etc., all at the same time, as we show in the example in Chapter 9. This is really the most practical way to do it these days, involving the least amount of masking and tape seams. Plus, if you’re painting in your own garage, there’s no rush to get the car out of the “booth.” You can leave it sitting overnight to fully dry with the doors and hood slightly open.
But what if you want the chassis and engine painted and detailed on your hobby car? Painting the body and frame separately (body-off-frame) is obviously the most effective and most arduous. Next comes removing the engine, painting and detailing it separately, painting the engine compartment while it’s out (possibly while painting the rest of the car), and then reinstalling the engine. But it’s not really that hard to get even show-quality chassis detailing without taking everything apart.
You have two choices: paint the chassis/engine first, and mask it all off while you paint the car; or, do all your messy bodywork, sanding, priming, and painting on the body first, then detail the under-carriage second. Doing it the first way is easier to mask. I’d do the messy sanding/bodywork/priming first. Then detail the chassis. Then, once the car is in position for painting, mask it from the rocker panels down to the floor, all around the car (this requires some creativity in the wheelwell areas). You can do this with masking paper, but it’s probably easier with plastic sheeting, such as Visqueen, taping it to the floor and all around the car. In similar fashion, you can “wrap up” the engine, as well as other under-hood components that shouldn’t be body-color (radiator, horns, wiring, etc.).
The biggest problem with today’s paints, in this regard, however, is pervasive overspray that seems to get onto and into places you’d never dream. That’s one reason why some painters like to mask a car down to the floor, all around the perimeter, even if the undercarriage isn’t already detailed (I had one instance where I found overspray inside my dash-board gauges!).
For this and other reasons, I prefer to paint the car first, then go back and detail the chassis, using mostly (if not exclusively) spray cans, which have far less overspray problem (besides being simple and cheap to use). You can tape masking paper along the rocker sills, either taping it to the garage floor, or just letting it hang down close to it. But a simple trick I discovered is to apply a coat of wax (car polish) to the entire outside of the car (windows, chrome, paint—whatever’s there). Then do all your chassis detail painting. When you’re finished, just buff all the wax off the car, and any overspray comes off with it—plus your car is waxed and ready to go to the show!
Finally, when masking, always err on the side of what you’re masking off, not what you’re painting. It’s much easier to clean or scrape paint off a window or chrome molding or piece of rubber than it is to repaint or touch-up a spot on the body that didn’t get painted because it had tape over it.
In general, as far as everything in this chapter is concerned, the primary job is to thoroughly sand, prep, and paint every square millimeter of the body that’s supposed to have paint on it. Once that’s accomplished, it’s relatively easy to clean up, detail, or even paint other areas—to the extent that you want them detailed.
Written by Pat Ganahl and Posted with Permission of CarTechBooks