Getting rid of rust through mechanical means such as sandblasting can be labor-intensive and costly. That’s why chemicals such as self-etching primers and rust converters are more effective and better options for restoring the appearance of rusty surfaces.
This article will compare self-etching primer vs rust converter and will show the right way. This will be based on how they work, application procedure, painting conditions and drying time, overcoating with primer and top coat, effects on human health, and other factors such as heat resistance, sanding, and rust level.
But first, here’s a table to highlight their key differences:
|Self-Etching Primer||Rust Converter|
|It’s used on surfaces with low to moderate rust.||It’s used on highly rusted surfaces.|
|It has a quicker curing and drying time.||It has a slower curing and drying time.|
|It requires a primer surfacer before applying a top coat.||It can be overcoated with paint and used as a primer.|
|Sanding directly after an application is not recommended.||Sanding can be done after application.|
|It emits harmful vapors.||It emits vapors that are not harmful.|
|It’s heat resistant up to a temperature of 200 degrees (Celsius).||It’s not heat resistant and shouldn’t be applied to surfaces in contact with heat.|
|You can clean materials used to paint with mineral acids or xylene.||You can clean materials used to paint with water and soap since it’s water-based.|
How Do They Work?
Self-Etching primer is a rust corrosion resistance coating that forms mechanical keys on a polished surface. This creates a base to allow adhesion to a non-epoxy primer followed by a top coat. It’s mainly used in the restoration and metal fabrication industry. You can use it on non-etched surfaces such as non-ferrous metals – steel, aluminum, copper, chrome, and fiberglass.
Its main ingredients are phosphoric acid and zinc. It works by the acid forcing the zinc on top of the metal. The zinc then galvanizes the metal to prevent exposure to water and moisture. Zinc phosphate, also present in the etch primer, serves as an anti-corrosion pigment. Phosphoric acid’s primary functions are:
- Etch the metal surface for the top coats to stick.
- It improves the durability of the top coat.
- It enhances the color hold-out of the top coat.
The Etch primer is available in three types:
- The first is the original type which you can use on any bare metal work – consists of two parts to be mixed.
- The second is the high build type for filling metal imperfections, so you don’t have to sand.
- The third is wield through type that provides durability and color hold out of your bare metal project.
On the other hand, a Rust converter is a water-based chemical that converts rust to a black inert substance you can paint on. It consists of two main ingredients called tannic acid and organic polymer.
Tannic acid is responsible for chemically converting the rust (reddish brown) into a more stable material called ferric tannate (black). Organic polymer’s function is to create a protective primer layer.
You can use a rust converter (also known as a rust reformer) on iron railings, boats, vehicles outside storage tanks, and sheet metal surfaces that are hard to sandblast.
Here’s how to apply the etch primer and rust converter:
Applying Etch Primer
- Wear eye protection gear, a respirator, and protective gloves.
- Mix baking soda in water to form a paste, apply over the rusted surface, then scrub with a brush and water to remove rust scales.
- Remove any grease with a grease remover and chemical stains with chemical removal agents on the bare metal surface you need to apply.
- Shake the primer spray can for a minute, hold it 12- 16″ from your workpiece, then spray in a uniform back-and-forth motion.
- You can spray a second coat after 30 minutes.
- Spray a heavy coat of primer surfacer or filler primer on top to remove any minor imperfections.
- Wet sand with 400 grit sandpaper.
- Repeat the spraying and sanding until you remove the blemishes and scratches.
- Allow it to dry and apply the paint over it.
Applying Rust Converter
- Scrub down any loose rust flakes, grease, and dirt with a wire brush to ensure the rust converter works effectively, and wipe it to leave a clean surface.
- Apply the rust reformer with a paintbrush, roller, or spray can, depending on the surface.
- Pour the required amount into a container when applying with a paintbrush
- Use the brush to apply coating on sections with rust.
- Let the paint dry and follow it up with a second coat.
- Clean up the brush and materials used with water and soap.
- Dispose of the remaining amount of rust converter and ensure you don’t return it to the original container.
Now that you know how they are applied, let’s cover their painting conditions and drying times.
Painting Conditions and Drying Time
You can apply a rust converter at a temperature between 50 -100 degrees Fahrenheit. Ensure you also don’t apply on a surface below 50 degrees Fahrenheit. Don’t use the product in direct sunlight or under damp conditions.
The rust converter takes 15- 45 minutes to dry to a smooth black finish. However, it takes 24 hours to cure. Applying a second coat allows proper conversion of rust to the black inert substance. It takes about 48 hours before you can apply oil-based top paint. It takes even longer if there’s humidity and dampness in the air.
You can best apply a self-etching primer at a temperature of 70-80 degrees Fahrenheit (21-27 degrees Celsius) and under a relative humidity of 50%. It dries to the touch after ten minutes, and you can handle the paint after 15-30 minutes. You can apply the top coat paint after 30 minutes.
The shorter drying time makes it a better option than the Rust converter. That’s why it’s often used in the car restoration industry to save more time working on other cars.
Winner: Self-etching Primer.
Overcoating with Primer and Top Coat
You can apply an oil-based primer over the rust converter, but it’s not a requirement. Once you paint a Rust converter on the rusted surface, you may leave it without applying a primer or a topcoat paint. However, if your surface is exposed to adverse weather conditions such as direct sunlight, rain, salt, and mist, it’s best to paint two coats of quality oil-based or epoxy paint over the Rust Converter.
Don’t use water-based or Latex paints to paint over the Rust Converter. Rust Reformer can also serve as an excellent primer under oil-based paint. It can adhere to painted or non-rusted surfaces but doesn’t provide additional rust preventive properties over those surfaces.
On the other hand, most Etch Primers need another coat of Filler/High building / Epoxy/Urethane Primer over it to fully seal everything up long term. The Urethane primer also acts as a barrier between the Self-etching primer and topcoat paint. Self-etching primers can be painted over in some cases but you’ll need to closely follow the manufacturer’s application and top coat guidelines.
Applying Self-etching primer on an epoxy primer or top coat creates a barrier between the metal surface and the Etch primer preventing it from performing its function. This eventually leads to the paint peeling off.
Self-etching primer requires a High-build primer to apply over it before you paint with a top coat. This translates to an extra cost of buying the primer. Rust Converter doesn’t require using a primer on top of it hence comes off as a better option.
Winner: Rust Converter
Effects on Human Health and Environment
The following are hazards to using self-etching primer and the precautions to take:
- It has a risk of damage to the eyes; hence wear eye protection gear and rinse with water once it comes into contact with the eyes.
- Wear respiratory equipment to avoid inhaling vapors that irritate the respiratory system and cause dizziness and drowsiness.
- Avoid disposal in water bodies since it harms aquatic organisms and may cause long-term effects.
The vapors emitted by a rust converter are not harmful unless in high concentrations. Rust Converter is also not hazardous to the environment. This makes Rust converter a better option if you are concerned about the impact on health and the environment.
Winner: Rust Converter
Other factors differentiating the Self-etching primer from Rust Converter include heat resistance, sanding over them, and the level of corrosion.
Self-etching paint can resist or remain unaffected by heat up to a temperature of 200 degrees Celsius. This makes it ideal to use on Automotive Water-Cooled Engines. On the contrary, Rust Converter can’t withstand high temperatures, so you can’t use it on a surface exposed to a lot of heat.
Sanding directly over self-etching primer is not recommended due to its acid-base nature. You can only sand it after applying a filler primer over it. However, you can sand Rust Converter if you see some toffee-colored areas. These are areas in which the Rust Converter hasn’t had corrosion to work on. You can do this before painting a top coat.
Level of Corrosion
The corrosion level is something to note before deciding which of the two to go for. Rust reformer is as reliable as it gets on highly rusted surfaces that won’t hold well on sanding and washing. Etch primers nip in the bud iron oxide on a moderate rusted surface.
There you go—Self-etching Primer vs. Rust Converter. Choosing one over the other is just a matter of preference. If you’re looking for a quick turnaround time and heat resistance properties, self-etching primer is the way to go. Rust Converter has the edge over Self-etching primer in that you can apply it on highly rusted surfaces without spending extra on primer surfacer and mineral acids (for cleaning). Healthwise, it’s also the safer option between the two.