There are several types of primers on the market. Manufacturers made them to perform one primary function – to help paint adhere to surfaces. Even though they perform the same functions, each primer is formulated differently to work on different materials. There are primers made for wood, some for leather, while some are specifically created for use on plastic, etc.

In this article, I’ll focus mainly on self-etching primers. Many painters swear by them, especially when it comes to painting metals. But what makes them so unique? What is a self etching primer?

A self-etching primer is a preparatory coating specifically formulated for use on metals. Manufacturers create it by combining an acid with zinc particles and a little pigment.

When you apply it to metal, the etching primer burns the metal’s surface and then embeds the zinc particles in it. As a result, the sleek metal gets enough “tooth” to grip the paint properly; hence the paint won’t peel off. Furthermore, it prevents more rust from forming on the surface.

Keep on reading to find out more about self-etching primers, where to use them, and how they differ from the other types of primers.

Self Etching Primer Vs Epoxy Primers

It isn’t easy to understand how self-etching primers work without knowing details about the other types of primers. You need to know all things about each to compare them and understand the best way to use them.

I’ll discuss everything you need to know about each type of primer and where to use it. I’ll also compare their pros and cons to help you make the best choice for your project.

• Self-Etching Primers

Etching primers are products created specifically for use on metals and fiberglass. They use acid to etch the surface, making it rough enough to accept paint. They also provide some corrosion protection, thus maintaining the metal’s integrity. 

Even though we classify them as primers, etching products do not offer the kind of protection that standard primers give. They prevent corrosion damage, but only for a little while.

For this reason, painters mostly use etching primers as a base before applying another primer with better protection capabilities. It’s like a metal sander that you use to scuff the metal in readiness for priming and painting.

Where to Use It

Self-etching primers are the product of choice for automotive body shops, especially when dealing with minor collision damages. They are perfect for doing color touch-ups on small areas, provided the car body has no dents.

If the body has a few dents, you need to apply Bondo – a putty for cars – before applying the primer. The reason is that etching primers do not provide long-term corrosion protection.

Therefore, if you putty the car body after priming, the exposed metal may start to rust as you wait for the filler to dry and set. As a result, you will have to go through the tiresome process of removing the rust again.

Therefore, it is best to save self-etching primers for minor repairs where you don’t need to use body filler first. This way, you will avoid the risk of rust forming on the surface before you finish the necessary repairs.

• Epoxy Primers

Epoxy primers work similarly to epoxy glues. They form a mechanical bond with the surface you’re prepping and the paint, essentially holding it in place.

This primer type is the most costly of all the primers, but it is worth it because it is also the most durable. You’re sure to get long-lasting results if you use the product as the manufacturers intended.

Where to Use It

Epoxies are also popular in auto body shops because they work perfectly on metals. However, unlike self-etching primers, they do not automatically etch the surface. You need to create some teeth by manually sanding the surface with sandpaper between 80 and 180 grit.

On the bright side, epoxy primers offer the best corrosion protection to metals. Therefore, if you have a dented car body, you must apply epoxy primer before you start repairing the dents with filler. The epoxy will keep the metal from rusting as you wait for the filler to dry and set.

Epoxy primer is the product that most car repairers prefer for extensive car body repairs. Such jobs require time and a lot of patience, and epoxies give you both without worrying about rust.

You can leave the epoxy coat bare for days and still pick it up from where you left off like it was yesterday. However, if you leave the primer bare for more than one week, you must scuff the first layer lightly before you continue.

It is an added step, but it is a small price to pay for being able to work at your desired pace.

• Similarities

The main similarity between these products is in their function – they all serve the same purpose even if manufacturers formulated them differently.

The purpose of a primer is to get surfaces ready for painting. However, each surface requires a different treatment, so the manufacturers create the primers differently.

For instance, metals are very sleek, and as we all know, paint doesn’t adhere well to smooth surfaces. The only way to make a smooth surface rough enough to paint is by scuffing it, but you can’t always sand metal.

Thus, manufacturers created self-etching primers with powerful ingredients to etch the metal.

On the other hand, surfaces like wood and concrete are very porous; therefore, they require products that can seal the pores to prevent them from absorbing the first layer of paint.

Plastics need a bonding primer to act as a magnet between them and the paint, and fabrics require a fabric medium to ensure that paint doesn’t bleed out when you wash them.

• Differences

The main difference between these primers is the ingredients used to make them. Self-etching primers are the only primers that have acid in their formula. They may work well on metals and fiberglass, but the acid will definitely have some adverse effects on organic surfaces like wood.

On the other hand, manufacturers make epoxies and other primers by dissolving a synthetic resin in a solvent. Some even add additives to increase the adhesion and protection capabilities.

Even though they use similar ingredients, the manufacturers combine them in different amounts, using differing techniques; therefore, they all have some unique properties.

Pros of Epoxy Primers

1. They are the most durable product among all surface primers; therefore, you can be sure that your project will last when you start it with them.
2. You can leave them bare for more than one week without adverse consequences.
3. They offer the best protection against rust, making them perfect for car repairs.
4. Epoxy primers are versatile because they are compatible with several surface materials. You can use them on metal, drywall, and even concrete.

Pros of Self etching Primers

1. They contain an acid that eliminates the need for sanding when preparing surfaces like metal and fiberglass for painting.
2. You can apply these products with minimal surface preparation. You don’t need to wash the surface – simply spray on the primer, and the acid will do the rest.
3. Etching primers dry super fast, allowing you to finish projects more quickly.

Cons of Epoxy primers

1. You must sand your surface before using epoxy primers. The sanding requires time and energy, but you cannot skip it because it directly affects the longevity of your paint job.
2. These products take too long to dry; therefore, your projects will take longer.
3. Epoxies are more expensive than the other types of primers.

Cons of Self etching primers

 

1. These products do not offer enough protection against rust. You must apply another primer over them to ensure the best results for your project.
2. Etching primers are not compatible with all surfaces or topcoats; therefore, you must always be careful as you shop for products to avoid buying those that will cause damage.

Should I Sand Self Etching Primer Before Painting?

Whether or not you sand the surface after applying the self-etching primer depends on the specific product you are using. You can paint over most self- etching products without sanding the surface first.

However, some products need some scuffing or sanding for the paint to adhere better. So how do you know whether to sand or not?

The only way to determine whether to sand your primer is to read the user’s instructions. If the product requires that you scuff it, the manufacturer will indicate it as they explain the best way to use the primer. 

If the product label does not mention sanding, you can achieve great results without it. However, you can still decide to sand it because sanding always helps the paint adhere to surfaces for a more extended periods.

How to Apply Self Etching Primer Properly Before Painting

Step 1: Surface Preparation

• Using mineral spirits, thoroughly clean the area that you wish to prime. This helps to eliminate all the dirt, grease, wax, and oil that may have settled on your surface. 
• Allow the surface to dry completely, then scuff the surface lightly using 400-grit. You can either dry sand or wet sand the surface depending on your preference but once you’re done, wipe down your surface till clean.

Step 2: Application

• Vigorously shake your primer can for one minute, ensuring you hear the mixing ball inside the can moving. If the mixing ball does not rattle, do not strike the can, instead, seek assistance from the primer provider or use another sprayer.
• Hold the primer container at least 12 inches from the surface and begin to spray in a steady back-and-forth motion, overlapping each coat slightly. Shake the can regularly as you use it and maintain the same distance to the surface as you spray while staying in motion.
• For the best outcome, apply at least two or three thin coats, allowing each coat to dry for at least two minutes before adding another layer.

Let the final primer coating dry for not less than 3- 4 hours before dry sanding. If you wish to wet sand the surface, let the last coat dry for only 15 minutes before sanding.

• Afterward, apply another thin coat of the primer.

Step 3: Clean Up and Waste Disposal

• After finishing the job, wipe the tip of the sprayer with a clean cotton cloth, then return the primer to storage. If the can is empty, ensure that you dispose of it properly. Do not burn or pierce the aerosol can, or you risk starting a fire.

Note:

• Do not use the primer on galvanized metals or on surfaces that exceed 93°C (200°F) when heated. 
• Avoid spraying in dusty or windy conditions, or you’ll inhale the product.
• Drying and recoat times are based on a temperature (21°C/ 70°F) and relative humidity (50%). Therefore, ensure that you check the weather conditions in your area before you start.

Can You Paint Straight Over Etch Primer?

There are a few etch primers that you can paint straight over, but I wouldn’t recommend painting directly on any of these products. All self-etch primers contain acid in them, but every manufacturer uses a different acid and uses it in different amounts. Some topcoats are compatible with these acids, while others are not.

The incompatible top coats always corrode and peel off the surface when in direct contact with the acid. On the other hand, the compatible ones are more resilient; therefore, they will hold out longer.

Despite their resilience, these products eventually give in due to the metal corroding as the protection of the etch primer wears out.

Compatibility aside, some etch primers contain more acid than others. As a result, they cause paint to deteriorate quicker – even the compatible ones.

You will have to refer to the user instructions of your product to know whether it is safe to paint directly over it or not. Manufacturers always know their products better; thus, their instructions are always meant to help you get the most benefit out of their product.

Even though the instructions will guide you, professional painters established a sort of  “silent rule” to always apply another primer over the etch primer to serve as a barrier between it and the paint.

Applying another primer eliminates the uncertainty of not knowing whether to paint directly or not. This “rule” has never failed before, and it has helped beginners and DIYers achieve great results.

The best products to use over etch primers are urethane primers. They are compatible with all top coatings; therefore, you won’t have to worry about the paint corroding or peeling off the surface. 

Can You Apply Epoxy Primer Over Etch Primer?

Whether or not you can apply epoxy primer over an etching primer depends on the brand of epoxy involved. Some epoxies work well with etching primers, while others cause the project to fail sooner than expected. If you want to know whether your product is compatible with etching primers, review the manufacturer’s label before using it.

Every manufacturer indicates whether or not it is safe to use their product on etching primers. These instructions often lead to great results but only to a certain extent.

I’ve encountered some epoxy primers indicated to be safe, but they failed after a little while. It showed me that you cannot bet on epoxies all the time; therefore, I discourage the use of epoxy primers over self-etching primers.

However, if you only have an epoxy, you can first test it out on a scrap piece of metal before starting your project. Spray a little etch primer on the surface, then follow with the epoxy to see how they react upon contact. This way, you will not waste too much time on a project that won’t last.

Can You Use a Self Etching Primer On Plastics?

No. Self-etching primers are only used for metal and fiberglass surfaces. This is because most plastic materials absorb self-etching primers causing them to lose their primary functional purpose. Let me explain.

The smooth surface of plastics makes it difficult for paint to adhere. You might think that self-etching primers will burn the surface a little to make it rough, but it doesn’t.

The product only sinks into the material instead of layering over it. When the primer sinks, it leaves the plastic bare with nothing to help paint adhere. Consequently, the color will peel off like it does when you paint over unprimed plastic.

Some painters, especially those who have never used etching primers before, do not know their effect on plastics. They apply the primer and paint over it as usual, only to be disappointed when the paint job doesn’t hold. For this reason, they always ask, what is the right product for priming plastic?

Plastic primers are the best products for preparing plastics because manufacturers design them specifically for plastic surfaces. They create the much-needed roughness on the material to allow the paint to adhere.

Alternatively, some companies create paints that grip plastic without needing a primer first. Products like Rust-oleum paints for plastic can stick to the material as long as you do some essential preliminary preparations.

First, you must clean the surface to remove dust and debris, then sand it lightly with 150-grit sandpaper to give it some tooth. As you sand, be careful not to go overboard, or you will ruin the surface and simultaneously spoil your chances for a flawless finish. After sanding, wipe away the sanding residue thoroughly, then apply the paint as usual.

3M Adhesion Promoter VS Self Etching Primer

 

Both self-etching primers and the 3M adhesion promoter are essential tools for working in a car workshop. Although they both help enhance adhesion, they have specific functions; therefore, you cannot interchange their uses without consequence.

The self-etching primer, for example, is best when you want to achieve a professional finish when painting vehicles, while the 3M adhesion promoter is best for installing automotive trims. Let me break down their information further to help you understand better.

• Purpose

In a car workshop, the self-etching primer is used to prime and etch the car’s surface. Manufacturers make it using acrylic lacquer, which helps improve the quality and smoothness of the final topcoat.

Secondly, it erodes the surface enough to make the paint stick to the surface. Additionally, some brands of self-etching primers have the added advantage of offering protection from rust.

On the other hand, the 3M adhesion promoter products serve a different purpose. For instance, mechanics use the “3M Automotive Adhesion Promoter” specifically for improving the adhesion of 3M foaming tapes and other 3M adhesion products on vehicles.

These products are necessary for making trims, bumper guards, and side moldings usually made using hard plastics.

• Places Used

You can use both self-etching primers and the 3M adhesion promoter on some surfaces, but there are others that you can only use either one or the other.

The 3M Automotive Adhesion Promoter works best on low surface materials such as Polyphenylene Oxide (PPO), Polypropylene plastics (PP), and ABS plastics. These are common materials used to make specific vehicle components such as interior trims, exterior trims, and the instrument panel.

On the other hand, the mechanics use self-etching primers on various surfaces, not all automotive-related. It will work well on any metal, fiberglass, or aluminum surface.

• Application

The 3M adhesion promoter and self-etching primers employ different application processes. Etch primers usually come in an aerosol can and can prime an area of about 12 square feet per can.

The primer is sprayed onto the surface and left to dry. Depending on the brand you’re using, it typically takes a self-etching primer 30 minutes to be dry to the touch and not less than an hour before you can apply the next coat.

Regardless of the primer used, however, it is recommended that you use it outdoors or in a well-ventilated room to prevent inhaling the toxic chemicals in the formula.

On the other hand, the 3M adhesion promoter is much safer; hence it does not require you to take the same precautions as its counterpart. You can apply it in an enclosed space without risking your respiratory health. 

Unlike self-etch primers, 3M adhesion promoters are applied using either a sponge or professional tools. The adhesive comes in packets that you open and use a sponge to apply. A single pack can cover approximately 150 square inches, and the product takes about 30 seconds to dry thoroughly.

• Which One to Use

Though both products are highly effective, you cannot use them interchangeably because they have specific purposes. For instance, you cannot use a self-etching primer to get a 3M acrylic foam tape to stick to low surface materials, and the 3M adhesive promoter cannot be applied as a primer on bare metals. 

The self-etching primer is for bare metal surfaces that require priming without having to scuff the surface. The 3M adhesive promoter, on the other hand, works best for jobs like installing side moldings or bumper protectors.

Conclusion

There are several kinds of primers on the market today, and their similarities make it challenging for some painters to differentiate them. They perform similar functions, but their different designs can’t allow you to use them on the same surfaces.

The most talked-about primers these days are self-etching primers. Many painters do not know much about them, so I wrote this article to answer the question,

What Is A Self Etching Primer?

A self-etching primer is a primer designed specifically for metals and fiberglass. They contain an acid that etches the surface to give it tooth to grip paint.

This primer is commonly used in auto shops to repair minor car body damages, but you cannot use it on its own. You must always apply a urethane primer over it before painting to give the metal better protection against corrosion.

I hope this answers all your questions concerning self-etching primers and the best way to use them. If you have any inquiries or additional information to share, please feel free to reach out in the comments section.