Last Updated on September 5, 2022 by Dave Farquhar
Whether you’re talking cars, computer modification, or restoring old toys, the subject of self-etching primer is bound to come up. You’ve probably seen it in the paint aisles with other spray paints and noticed it costs a little more than regular primer. But what is self-etching primer, and when should you use it? And where’s the best place to buy it?
- Self etching primer roughens the surface of metal, helping it to bond better
- On metal surfaces, self etching primer gives a tougher and more durable finish than regular primer
- You do need to remove any old paint or rust before using self etching primer
- Self etching primer doesn’t provide any rust protection, so either apply a topcoat quickly, or protect your project from rain until you can paint it
- Most stores that sell spray paint also carry self etching primer
What is self-etching primer?
Self-etching primer is primer intended for fiberglass surfaces and metals. It contains phosphoric acid and zinc. The acid etches the surface and deposits the zinc, giving itself a slightly rough surface to grab onto. This gives the primer a chance to bite and promote maximum adhesion, without you having to sand the surface to roughen it beforehand. Self-etching primer can save you quite a a bit of time since it saves you the step of sanding your surface, and it cures quickly.
Rust-oleum self-etching primer cures in about 15 minutes, so it can really speed up a paint job.
When should you use it?
Use self-etching primer on bare metal. It will work on just about any metal, whether steel, diecast alloys, pot metal, or others.
The caveat with this type is that you need to remove the old paint and treat the rust ahead of time. If you need to use any fillers like Bondo to make spot repairs, apply the fillers before you apply your primer. It also requires a clean surface. Dirt or oil residue interfere with the priming process. Good surface preparation is always key to a good paint job.
Self-etching primer is especially effective on diecast, which ordinary paints and primers sometimes have trouble sticking to. If your diecast has a lot of fine detail, the self-etching primer will get into the areas that you can’t sand and stick anyway, and it’s thin enough that it won’t hide detail like some primers will. It provides excellent adhesion and won’t scrape off without a lot of effort.
I like it on old tinplate trains too. Self-etching primer adheres to tin-plated steel really well and it dries to a somewhat rough finish that makes it easy for the top coat to grab onto.
A note about self-etching primer and aluminum
Aluminum is especially difficult to paint, partly because oxidation forms on its surface so quickly. You need to use self-etching primer to paint aluminum, but prep work is especially important. Unlike with other metals, you do have to sand the aluminum first, with 120-grit sandpaper. Follow up by wiping it down with mineral spirits. If you don’t prep the aluminum well, the primer scratches off rather easily.
What self-etching primer doesn’t do
Self-etching primer doesn’t provide a good rust barrier, so if you use it on steel or another ferrous metal, you need to protect it until you spray another coat. You can follow the self-etching primer with a high-build primer to help with surface imperfections, or if you’re relatively happy with the surface, with a top coat.
If you’re painting something with fine detail, don’t use high build primer. Follow up with regular primer, the same brand as your self-etching primer. After it dries and you’re happy with the results, continue with your topcoat.
Let the topcoat dry thoroughly, and you’ll be rewarded with a durable, professional finish.
Caveats with self-etching primer
The most important thing to remember when using this type of primer is that your surface needs to be clean and dry. Oil from handling the item with your hands will interfere with the primer. Clean the surface with mineral spirits and let it dry for a few minutes before spraying it.
With most metals you don’t really have to sand beforehand, but with aluminum you do.
Advantages of self-etching primer
Despite the caveat, self-etching primer is really nice to work with. It dries fast, and tough. It only needs 15-30 minutes to dry to the point where you can handle it, and you can spray a coat of paint on it in 30 minutes, which is half the time most other primers recommend.
After 30 minutes of spraying an old junk diecast car with it, I picked it up and tried to scrape it with my fingernail. It discolored the primer, but it didn’t come off. So even though you need to spray a top coat over self-etching primer in a hurry, it gives you durable results in a hurry. That’s really nice for hobby use involving small projects. You can prime and paint one day, then reassemble the following weekend after letting the parts sit all week.
I also noticed it didn’t obscure any detail, or imperfections for that matter. Whether that’s an advantage or a disadvantage depends on your application. When restoring or customizing toys, thick paint is usually a disadvantage, so self-etching primer helps. Makers of vintage toys didn’t use primer at all, but that’s one of the reasons old toys frequently have such beat-up paint jobs. It was also easier to get away without primer when paint had lead in it. By using a self-etching primer, you can get a stronger finish with one thin coat of primer and one thin coat of paint and lose less detail than the original paint job did.
Like other types of primer, the topcoat didn’t run when I applied it. When I repaint toys without a primer, the biggest problem I find is the paint running, so I end up with inconsistent colors at the top, and pools of paint at the bottom. To correct it, I end up having to apply multiple coats.
After you apply your topcoat, I do recommend you let everything dry for a week before putting it together and into use, if possible. It will last longer.
Where to buy self-etching primer
I buy it at home improvement stores, where a can costs about 75 cents more than regular primer. The convenience and durability is worth 75 cents to me. If you don’t have a home improvement store near you, you can also get it at auto parts stores. It costs quite a bit more than house-brand primer from the world’s largest discount chain, but the results are worth it to me. I’m sure you’ll like the results too.
Normally it comes in gray, but I accidentally bought it in olive drab once. Olive drab works nicely if you’re going to paint it any shade of green. You can also use it under red, if you want the red to be a bit darker than it would be otherwise.
What is self-etching primer, in conclusion
Self-etching primer is no secret at all in automotive restoration circles, but it doesn’t get a lot of talk in other circles, for whatever reason. That’s a shame, because self-etching primer is great stuff. It lets me get better results in less time, which is pretty much the goal in anything. The results look better and they’re more durable.
Nobody likes watching paint dry. So whether you’re restoring old toys or trains, custom painting a computer case, restoring other worn-out metal household items, or fixing up a car, this stuff may very well be the best trick you’ve heard of in a long time. I can’t recommend it highly enough.
One thought on “What self-etching primer is”
Nice simple explanation which has helped me decide on how to paint a steel and aluminium case.
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