The second—and usually better—option is to thoroughly sand whatever paint is on the car, then cover it with a good sealer followed by a modern catalyzed primer. Actually, if the existing surface sands down nice and smooth, you could spray a coat of catalyzed sealer over it (preferably a colored sealer of a shade close to the final paint color, if the sanded surface ends up multi-colored), followed by the new paint. However, most cars needing a paint job also have dings, door dents, scrapes, or other surface problems that won’t simply sand out. So most often, if you’re going to paint over existing paint, we suggest sanding it down with relatively coarse paper (180 to 220 grit), and then either shooting the whole body (preferably), or just any rough, dinged, or bodyworked areas, with a good, catalyzed, high-fill primer. If the paint on the car is multilayer, old, or otherwise edgy, give yourself extra insurance and add a coat of sealer before the primer. Then you can use some catalyzed spot putty where necessary over the primer and start block sanding, as we detail in following chapters. Most of my early paint problems, when painting over existing finishes without stripping, occurred because I was using lacquer primer and lacquer paint. We talk more about this later, but lacquer solvents are extremely aggressive, and lift or wrinkle all kinds of underlying paints, especially older non-catalyzed ones, including old lacquer. For both of these reasons, modern paints really are better, especially if you’re spraying it over existing paint.
Here’s a similar car that has been sanded and partly stripped, ready for new paint. Assuming it’s all straight and smooth, I’d spray some sealer on any bare metal spots (if not the whole car, just to be sure), and then mask and spray color. This is exactly what we did with several cars as teens (short of taking glass out). We then had them sprayed by a local painter, with never any problems. With your own garage and equipment, you can do the painting yourself.
Here’s that Nomad again. As we say in the text, there’s no point stripping or grinding out someone else’s filler (undoing work already done) if it’s done relatively well. You can see from the waves in the reflection in the side of this car that (1) it has filler in it, and (2) it needs further block/board sanding to get it as straight as it should be. But the paint’s been on the car a good while, and there’s no rust coming through, no cracks, and none of the filler is falling out (which does happen). We’d recommend sanding with 80-grit on a long board until it’s pretty straight, then spraying with high-fill primer and blocking again with 180-grit.
The third option is iffy and always debatable. It pertains to older vehicles, or ones that you know have been damaged and bodyworked. But in this case the exterior surface, whether it’s fresh and shiny, old and faded, or maybe in a coat of primer, looks relatively smooth and straight and shows no evidence of cracking, bubbling, rust, or other badness. If the surface is shiny and fresh, and you want to repaint it, I can only assume you just bought the car and paid for a paint job you didn’t want. Don’t compound the issue (in my opinion) by immediately stripping this paint off only to find what you consider to be an excessive amount of filler underneath. Lots of good paint jobs, even by big-name builders, have filler under them. It’s the most expedient way to get a super-straight show-winning body and paint job. But the majority of these cars are stripped to bare metal to begin with, metal-worked pretty close, and then the filler and other undercoats are added properly. If you strip all this off, you’re just erasing several man-hours of work that have to be repeated, either by you or someone you’ll have to pay big bucks by the hour. If somebody has already spent a lot of time bodyworking, priming, and sanding the car, you’d be nuts to strip all that out and do it over again, right? Now, if there’s rust under there, or the filler’s an inch thick, it’s a different story. The body needs stripping to start, and more work after that.
On the roof of this car, however, we see that the paint is crazed and checked. Assuming only the top coat of paint is so affected, it either has to be completely sanded off (down to stable undercoats), or strip just the roof of the car, either by media-blasting or liquid hand-stripping. If the checking isn’t too bad, you might be able to sand it smooth and coat it with today’s catalyzed high-fill primer. But often such checking will “telegraph” (through shrinkage) back to the surface, especially after new paint is rubbed out.
How can you tell if a car has filler in it, especially if it’s painted and looks smooth? The first clue is to check the backsides of any panels accessible to see (or feel) if they’re wrinkly or wobbly. Some people can “hear” filler just by rapping their knuckle along the outer body. Otherwise, I have seen numerous “filler finder” devices that use spring-loaded magnets, or are battery-powered with lights or beepers. However, a simple, small refrigerator magnet, like the two shown here, can work just as well, especially if you use the same one regularly and get to know its “feel.”
Let’s use my ’52 Chevy as an example of how you can use high-fill primer. We know this car only has factory paint, except where I’ve spotted it in with lacquer, such as here on the trunk, where I removed the handle/emblem, welded up the holes, ground them, then added a thin coat of filler and sanded it smooth before priming, spot painting, and rubbing it out. It looks okay here, but I could still see ripples in it after a few months’ shrinkage.
Some sanding with 180-grit paper on a long board quickly revealed low areas. But they’re not deep enough to require new filler. In the lower panel you can see two small high (bare) spots that uncovered themselves from this block sanding. I tapped them down level with a body hammer and dolly; they won’t require filler either.
Since I know this paint surface is stable, I sprayed it with a few coats of catalyzed, high-fill primer.
Again using the long board with 180-grit and sanding at 45-degree angles, both left-to-right and right-to-left on this curved surface, it didn’t take long to level the area, using only the primer and existing paint as “filler.” If you hit bare metal during this process, it’s best to spray these spots with an etching sealer, or at least more primer, before spraying base color.
But this option we’re discussing pertains to vehicles that you know or suspect have had some bodywork done, and you assume it has been done properly because nothing indicates otherwise. In such cases I suggest not stripping the car because most types of stripping either remove existing filler, or “infect” it with chemicals so it must be removed. Opinions differ on this issue, but I think it’s smarter to be an optimist. Given that you’ve checked carefully for any real gremlins, and the body looks good the way it is, sand it down and repaint it. Even if it’s a little wavy, do your block sanding on what’s already there, if it’s a stable surface. Add some high-fill primer or spot putty, as needed.