What is a Self-Etching Primer?

Primer is 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 primers are less likely to stick correctly to these surfaces.

This is when you need 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. 

Why Self-Etching Primers?

The reason why self-etching primer works so well with fiberglass and metal is because it contains components such as zinc and phosphoric acid. 

These components will etch or affect the 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. 

The combination means less chance of the primer slipping off the surface of the metal or fiberglass. 

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. 

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.

The main difference is that filler primer, unlike self etching primer, contains no acid or other etching ingredients like zinc.

Typically, you would 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 of sanding, which means far less effort and job done quickly.

The self-etching primer should also 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 fully. 

What is Self-Etching Primer Used for?

While self-etching primers are used in many applications, they are arguably most often found in the automotive industry. 

In general, self-etching primer is designed to be used on fiberglass and metal surfaces, including aluminum, brass, steel, and more. 

While primarily used on large flat industrial surfaces, it also does quite well in tight areas, especially the little nooks and crannies that may be found on some items.

For example, a self-etching primer is ideal for painting small models, figurines, or metal-cast figures. In addition to priming and etching the surface, some products can protect the materials from corrosion and rust. 

Another material that self-etching primer is well-suited for is hard plastics. Such surfaces often tend to be slick and challenging to paint. 

However, the self-etching primer will rough up the plastic just enough to allow the primer to stick. This allows you to paint over the primer and get the desired results. 

One word of caution is that not all self-etching primers are suited for hard plastics. Be sure to check the label before purchasing any self-etching primer. That way, you’ll know the exact materials it is designed to work on. 

Will Self-Etching Primers Work on Wood Projects?

Yes, but you do not essentially need to use a self-etching primer if you have other specific options available.

That is because, unlike metal, fiberglass, or hard plastics (which are not nearly as porous and require the surface to be roughened), the wood surface is porous and rough enough to take a standard primer. 

And given the general prices between standard wood primer and self-etching primer, you may be paying more than you should achieve the same results. 

The self-etching primer will work on wood if no other primer is available. However, there will be no difference between that and standard primer. 

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 fully. That’s it!

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. 

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 second primer with these 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 doing the sanding yourself only takes a few minutes.