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. 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.
Typically, you would sand or rough the surface of the metal or fiberglass to get the primer to stick. But by using a self-etching primer, you skip that time-consuming step.
It also means far less effort as you only need to apply the self-etching primer with no sanding required.
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 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.
The self-etching primer should not be confused with acid etch. While similar in some ways, acid etch does not contain any primer.
Self-Etching vs. Regular Primer
The main difference is that standard primer contains no acid or other self-etching properties like zinc.
This means that if you only have standard primer, you will need to rough the surface of the fiberglass, metal, or hard plastic using a sander.
Otherwise, self-etching and standard primers are quite similar in their function.
Each protects the underlying surface and provides a sticky platform for the paint to adhere to. You should always read the label to fully understand what each product does.
What is Self-Etching Primer Used for?
Self-etching primer is designed to be used on fiberglass and metal surfaces. And it can work on most types of metal, including aluminum, brass, steel, and more.
While primarily used on flat surfaces, it also does quite well in tight areas, especially the little nooks and crannies that may be found on some items.
If you are painting metal-cast figures, for example, self-etching primer is ideal.
Another material that self-etching primer is well-suited for is hard plastics. Such surfaces often tend to be slick and challenging to paint.
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.
When to Use Self-Etching Primer for Your Project?
You can use it on any metal, fiberglass, or hard plastic surface suitable for painting.
Simply apply the self-etching primer first, allow it to dry fully, and then apply the paint.
You can use it on small projects or large surfaces that are flat or have many nooks and crannies.
In addition to priming and etching the surface, some products can also protect the materials from corrosion and rust.
While self-etching primers are used in many different applications, they are arguably most often found in the automotive industry.
The combination of large and small metal parts makes this type of primer an ideal choice.
However, this primer can be used for many projects, including large sheet metal surfaces down to small models and figurines.
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?
Yes, but you do not need to use a self-etching primer. That is because the wood surface is porous and rough enough to take a standard primer.
It is unlike metal, fiberglass, or hard plastic, which is not nearly as porous and requires the surface to be roughened.
If you have no other primer available, the self-etching primer will work on wood. However, there will be no difference between that and standard primer.
And given the general prices between standard and self-etching primer, then you may be paying more than you should achieve the same results.
Can Self-Etching Primer Be Used on Top of Paint?
It is not recommended to apply self-etching primer over paint. The acids in the primer will harm the paint underneath.
This may cause the paint to chip, crack, or simply fall apart.
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.
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 primer dries, you can add regular paint on top. The acid in the self-etching primer will have dried and no longer be a danger to the paint.
So, if you have a painted surface and you wish to preserve the paint, then do not apply self-etching primer over the top.
You will only find yourself having to scrape off the remaining paint and primer and start all over again.
Can Epoxy Primers Be Used Over Self-Etching Primer?
That’s going to depend on the brand. There are versions of epoxy primers that work well with self-etching primers and will say so on the label.
Others may not work as well, but it is possible that epoxy primers that do not have any information on their labels may still combine with self-etching primers.
It is true that some manufacturers of epoxy primers do not recommend that their product be combined with self-etching primers.
That is what the label is for, so be sure to read it before making a purchase.
In addition, some epoxy primers may work, but only if you take certain steps. This way, proper adhesion can be achieved.
If you are still not sure, it is best to use scrap material first to act as a test. Apply a coat of self-etching primer and then follow it with epoxy primer.
If it sticks, then you can proceed. If not, then you will need to find another product.
Does Self-Etching Primered Surface Need Sanding Before Painting?
Once the self-etching primer is applied and it has dried, you should be able to paint directly on the surface without having to sand.
However, if the self-etching primer you use does not have any products that protect against rust or corrosion, you may want to apply a second primer with these qualities.
That will help protect the surface underneath from any rust or corrosion.
The only time you may want to sand after applying the self-etching primer is when you see the paint is still having difficulty sticking.
In such cases, a light sanding can roughen the surface just enough to let the paint stick. But for the most part, that should not be necessary.
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.
This means that when you have metal surfaces that need to be painted, 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.
It may be for some metal, fiberglass, or hard plastic surfaces that self-etching paint is not the best alternative.
For small flat surfaces, you may want to simply take a minute or two to rough it up with sandpaper and then apply a standard primer or one with rust prevention elements.
If you do not have any self-etching primer, it may not be worth purchasing if doing the sanding yourself only takes a few minutes.
However, not all small objects are well suited for sanding. It may be impossible to properly sand and roughen the surfaces for metal figurines and other objects with many nooks and crannies.
For larger flat surfaces or metal, fiberglass, or plastic difficult to sand, self-etching paint is the obvious answer.
Be sure to either purchase self-etching primer that includes rust and corrosion preventative elements or have such products ready.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for more than a decade to serve the customers in areas such as Charleston, Mount Pleasant, Beaufort, Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques that are being implemented in his profession. You may read more about him here or get in touch with him here.
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