Why You Should Not Apply Undercoat To Rust

Rust on the undercarriage is a nightmare as it signals many wrong things with your car, and the worst part is when you don’t discover it on time.

Many car owners use undercoats to prevent and stop rust from undercarriage.

Still, there have been opinions on where undercoats and rust are concerned, including whether the former sticks to the latter.

Quick Answer

As a whole, the undercoat will stick to rust instead of the metal and eventually peel off from the undercarriage. Spraying an undercoat on the undercarriage without dealing with rust isn’t the perfect solution. It could make the rust situation worse and make the car unsafe.

Spraying an undercoat without dealing with rust is one mistake you shouldn’t make when treating or preventing rust from your car.

What should you do then?

Will Undercoat Stick To Rust?

There’s no debate that rust is bad for your vehicle, and the more you leave it on the undercarriage, the more you risk the car’s structural integrity.

Coating your car’s undercarriage will help keep the rust away from your car, but what happens when the rust is already there?

Quick Answer

As a general rule, an undercoat will stick to rust as most undercoats provide adhesion to a metal surface, and with rust present on the metal, the product sticks to the rust instead. 

Undercoats have films that, when dried on the surface, turn hard, arid, and brittle and bond to rust.

In addition, it traps the rust and the moisture and air, causing corrosion beneath and worsening the situation. 

Eventually, the bonded rust will flake off to reveal a deteriorating undercarriage caused by rust.

Poorly applied undercoats don’t deal with the rust but cover it up, trapping and worsening it.

Most undercoats will do a great job of making the rusted surface sharp and good outside.

For example, a rubberized undercoat will make the outside look great while rust spreads underneath. That’s a ticking time bomb.

If undercoats stick to rust, their purpose of removing rust has failed.

Removing rust before applying an undercoat on the undercarriage is more appropriate. 

You must also neutralize the rust before proceeding with undercoating.

Undercoat sticking to rust isn’t a great thing, and you should prevent that from happening. 

If you want to stop rust from worsening on the undercarriage, here’s what to do instead to prevent the undercoat from sticking to the rust: 

Scrape The Rust Away

You must remove rust from the undercarriage to prevent the undercoat from sticking to the surface.

Take precautions when doing this. 

Ensure your car is elevated over a flat surface, and protect your eyes and nose from rust dust.

If it’s deep rust, you may have to take the vehicle to an auto shop to replace the car part.

Apply Degreaser

You shouldn’t power wash or use some chemicals in the undercarriage, so you’ll have to mask them off before applying a degreaser.

Cover the brake assembly, exhaust system, transfer case, and transmission, and maybe remove the wheels to access the wheel arches.

Then, degreaser on the exposed surfaces in the undercarriage to remove rust flakes, debris, oil, and wax to ensure the treatments you use are effective.

Pressure wash the undercarriage

Then, pressure wash the undercarriage to remove rust remnants, dirt, and debris from the unwrapped surface and hard-to-reach spots, then leave it to dry. 

Prime The Undercarriage Using A Rust Converter

You need to reinforce rust prevention using a rust-inhibiting primer on the surface.

It’ll also cover spots you missed and make them safe and ready for undercoating.

You may have to apply multiple layers of primer over the undercarriage to cover everything.

Ensure you read the instructions on the primer to know how long to wait between each coat.

Spray Rust Reformer

A rust reformer on the rusty surface will contain the existing one and prevent future formation.

You should spray a rust reformer before coating the car. Spray it over the exposed areas, especially the brake and fuel lines.

You may need to apply multiple coats, therefore, read the instructions on the package to know how long to wait between coats.

Apply The Undercoat

After removing rust and treating the surface, the undercarriage is ready for an undercoat.

This is only when the undercoat won’t stick to the rust and will last more effectively.

Furthermore, it will prevent future rust attacks from the undercarriage.


Using an undercoat over rust isn’t the best option when the rust is still on the undercarriage.

You must remove the rust and treat the surface to neutralize budding corrosion on the surface. 

Then, apply layers of rust-inhibiting primer and rust reformer for effectiveness.

Then, ensure you monitor the undercarriage occasionally to check for rust and carry out routine maintenance to prevent rust.

Key Takeaways:

  • The undercoat contains adhesives and sticks to rust.
  • Spraying an undercoat over rust will enable rust growth underneath.
  • You must remove rust before undercoating by scrapping, using rust inhibitors and rust reformers.