Paints and primers are relatively easy to apply to wood because the surface is porous enough to allow the product to get inside.
However, fiberglass and metal have surfaces that are not as porous. This means that regular primers are less likely to stick correctly to these surfaces. This is when you feel the need for a self-etching primer to do the job.
A self-etching primer is a product that does two things. First, it acts as a primer, and second, it etches the metal (or fiberglass) surface so that the primer itself can adequately stick.
This means you can get the protection needed while accomplishing the task in fewer steps. The result is taking less time to apply the primer and paint to protect the surfaces of metal and fiberglass.
Self-Etching vs. Regular Primers
Self-etching and standard primers are pretty similar in their function. Each protects the underlying surface, fills in any minor surface pits, and provides a sticky platform for the paint to adhere to.
However, the main difference lies in their ingredients. Unlike self-etching primer, standard filler primer contains no acid or other etching ingredients like zinc.
Typically, you would need to sand or rough the surface of the metal, rigid plastic, or fiberglass to get the standard primer to stick using a sander. But by using a self-etching primer, you skip that time-consuming step, which means far less effort and the job done quickly.
That said, the self-etching primer should not be confused with acid etch. While similar in some ways, acid etch does not contain any primer. Before buying and using any of them, you should always read the label to understand what each product does and how.
How Well Do Self-Etching Primers Work?
The reason why self-etching primer works so well with fiberglass and metal is that it contains components such as zinc and phosphoric acid.
These components will etch or affect the smooth surface in such a manner that the primer will stick better. And when the primer sticks better, the paint applied over the primer will also stick better.
In addition to etching the surface, some self-etching primers contain elements that reduce and even stop oxidation from occurring. This means rust and corrosion are less likely to take hold of the protected surface.
However, not all self-etching primers have these additional elements, meaning the metal is no more protected than it would be under a simple coat of paint. So, if you need such protection, be sure to look around and pick the product wisely. Also, be prepared to invest a bit extra as these types of primers come at an extra cost.
What is Self-Etching Primer Generally Used for?
In general, self-etching primer is designed to be used on fiberglass and metal surfaces, including aluminum, brass, steel, and more. But they are arguably most often found helpful in the automotive industry.
Primarily used on large flat industrial surfaces, self-etching primer also does quite well in tight areas, especially the little nooks and crannies that may be found on some items like models, figurines, or metal-cast figures.
Another material where self-etching primer is well-suited is hard plastics, as these surfaces tend to be slick and challenging to paint. The primer will rough up the plastic just enough to allow the primer to stick and allows you to paint over it quickly to get the desired results.
Remember that self-Etching primers will also work on wood projects, but you do not necessarily need to use them if you have other specific options available at hand. There will be no difference between the self-etching primer and the standard primer when used on wood.
Unlike metal, fiberglass, or hard plastics, the wood surface is porous and rough enough to take a standard wood primer. And given the general prices between a regular primer and a self-etching primer, you may be paying more for achieving the same results.
Etch Primer vs. Wash Primer – Are They the Same?
Self-etching and “wash” are the two terms that generally mean the same thing. The common idea is to etch the metal on the application.
Put simply, a wash primer is a synthetic product that normally contains phosphoric acid or zinc chromate. It is applied as the first coat on metal surfaces.
The primer normally has anti-oxidation and anti-corrosive elements. This means that once the primer dries, the metal surface is protected, and a layer of paint can be applied over the primer.
The goal of the wash primer is to make the entire painting process go faster and more efficiently while protecting the metal itself from rust and other corrosive properties.
Any metal surface vulnerable to rust or corrosion is a prime candidate for a wash primer. These mostly include aluminum, brass, stainless steel, and galvanized metals.
Applying Self-Etching Primer – Things to Keep In Mind?
Applying a self-etching primer like Rust-Oleum is not tricky, as it’s available in easy-to-spray aerosol cans.
Simply shake the can, apply the first coat, and let it dry and cure thoroughly. Once the self-etching primer is applied and has dried, you should be able to paint directly on the surface without having to sand.
The only time you may want to sand lightly after applying the self-etching primer is when you see the paint still has difficulty sticking.
If that’s the case, a light sanding can roughen the surface enough to let the paint stick. But for the most part, that should not be necessary unless it’s mentioned on the product that sanding is recommended.
Also, remember that if the self-etching primer you use on metal does not have any additives to protect against rust or corrosion, you may want to apply a primer with rust-inhibiting qualities. That will be good to help protect the metal surface underneath from any rust or corrosion.
Can Self-Etching Primer Be Used on Top of Paint?
Applying self-etching primer over existing paint or any other clear coat is not recommended because the acids in the primer will harm the finish underneath, which may cause the paint applied, to chip, crack, or simply fall apart.
The purpose of self-etching primer is to be applied directly to the metal surface. The acid will etch or scar the surface just enough to allow the primer to stick.
Once the acid in the self-etching primer dries, it’s no longer a danger to the paint, which means you can add regular paint on top. But in cases where the paint is the base coat and primer is added, the damage caused by the acid will cause the underlying paint to flake away.
Can Epoxy Primers Be Used Over Self-Etching Primer?
That’s going to depend on the brand and the steps you take for proper adhesion. There are versions of epoxy primers that work well with self-etching primers and will say so on the label.
While many others may not work as well, it’s still possible that epoxy primers that do not have any information on their labels may still combine with self-etching primers. That’s what the label is for, so be sure to read it before purchasing if you desire to do so.
If you are still not sure, use scrap material first to test. Apply a coat of self-etching primer and then follow it with epoxy primer. If it sticks, you can proceed. If not, you will need to find another product.
The Bottom Line
So, now you know about self-etching paint. You know what it is for, when to use it, and how it mixes with other products.
Typically, when you need to finish difficult-to-sand fiberglass, plastic, or smaller metal surfaces (like figurines, automotive parts, and objects with many nooks and crannies), you can save considerable time and effort by using self-etching paint.
However, there are times when you may not want to use self-etching paint, like on wood, smaller flat metal surfaces, or larger sheets of metal that can easily be sanded. In other words, it may not be worth purchasing a self-etching primer if sanding only takes a few minutes.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for over a decade serving customers in areas such as Charleston, Mount Pleasant, Beaufort, and Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques implemented in his profession. You may read more about him here or get in touch with him here.