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Classic Car Underseal


The underside of a classic car rarely gets admiring glances and is often neglected, out of sight out of …. you know.

Road salt, mud, and moisture all play their part in the silent slow destruction of your cherished classic, it needs to be stopped, now!

Car under-sealer is a thick, textured, and flexible under-body coating. It can be applied by brush but best with a spray gun. The under-sealer protects the metal from flying debris, salt, and moisture. It also helps suppress tire and road noise from entering the cabin.

I recently under-sealed a VW Corrado, I’ve included some of the pictures which gives you an idea of what’s involved. Some people call it under-sealer others undercoating, either-way it’s a dirty job, but it really is worth the effort.

It not only protects but actually makes your classic more pleasurable to drive, the thick coating reduces excessive cabin noise usually associated with older cars. I wrote an article about some other useful ways to reduce cabin noise, check it out here “How to make a classic car quieter”.

How Do You Underseal A Car?

For best results, the under-body needs to be spotless. Not much point is spraying two thick coats of protective sealer over the muck, moisture, or worse, road salt. This would cause the chassis to corrode at an even faster rate, the moisture is now locked in.

The summer is the best time to underseal your classic, a dry warm ambient conditions are ideal for under-sealing. Yes, I know the winter suits because the car is off the road anyway, but damp cold weather is going to cause problems down the road.

We can avoid shooting lead in our toes if we take extra care at the preparation stage. This like all painting jobs is crucial.

Power Wash

I recommend getting the chassis, mechanicals and wheel arches power washed, hot preferably, and while it’s on a car lift. Not every garage offers this type of service, so you may need to make some inquiries.

For sure your local classic car club will know somebody. Yes, you could do it yourself from axle stands but the constant struggle maneuvering the lance to get all the angles covered is a right pain in the jacksie and you’ll get wet to your naks, trust me.

After washing and allowing it to dry out thoroughly, next comes the inspection stage. You’re on the lookout for rust holes and areas of heavy corrosion. You’ll need a car jack, axle stands, creeper, safety glasses, inspection light, and a long flat head screwdriver for probing.

Obviously, if you have a 2 or 4 post car lift, the whole job’s going to be a pleasure.

If you’re lucky, you’ll only have rusty patches to deal with, but if you find rust holes, they’ll need to be patch welded.

Classic Car Welding


You’ll need to make any repairs to the chassis now before under-sealing. I use a Mig welder, with Argon as a shielding gas, you could use flux core wire with no gas, but the gas makes a cleaner weld. The Mig is a fantastic tool, it’s easy to use and after a few tests runs you’ll be pretty good.

Lots of guys I know use Tig, and it is a pretty weld with less panel heat, but it takes a lot longer to master.

Definitely check out an article I wrote on different types of welding and how difficult they are for a beginner to master “Is Welding Hard To Learn”.

A few helpful tips I have learned when Mig welding a patch panel:

  • Use the same gauge patch as the panel you’re welding to
  • Cut the rust hole to suit the new patch, not the other way around
  • Use a reciprocating saw to cut the panel, cuts neater than a whizzie wheel
  • The tighter the patch fit, the better the finish
  • Clean a really good ground close to the welding area
  • Clean the welding area really well
  • Test weld on a scrap of the same gauge metal to adjust amps and wire speed
  • Spot weld in alternate corners, avoids heat and distortion
  • Hammer each spot weld directly after welding, less grinding equals a stronger panel

Wire Brush

Rusty patches will need a rub of a wire brush. I use wire brush heads on a drill. They come in all shapes and sizes, but you’ll need a selection to suit those hard to get places.

You’ll need a good mask, I use a 3M rubber mask, anything else fogs up the safety goggles, sooo frustrating, I also use disposable ear protectors.

This is the most important stage of the process, cleaning any loose sealer and rust from the chassis, sub-frames and axles will give the undercoating a good solid surface to bind to. The better you prepare the surface the longer the coating will last.

Areas that are not bad enough to consider a patch panel but are more advanced than just surface rust will need to be treated before under-sealing.

Rust Converter

I use a rust converter, it’s exactly what it sounds like, it’s a liquid chemical designed to neutralize rust, there are many good brands and all do the same job. I use Rust-Oleum when I can get it. You can check it out here on Amazon.

Application is easy, remove loose rust with a wire brush and apply 2 coats of the converter using a brush or it’s available in an aerosol spray. I prefer the brush method as I can get a flow of chemicals into affected metal seams.

After several hours of drying time, treated areas turn black, after 24 hours the area can be over-painted.

Final Prep

Now we’re ready for the under-sealing. I don’t under-seal in the damp, a hot dry day is best, otherwise, we’re locking in moisture, the enemy.

If you are overprotective of your garage floor or driveway, cover it down with cardboard or plastic sheeting. Any over-spray on the body can be removed easily with some gas on a rag.

Spraying outside is OK, as long as the weather is fine, if you are spraying indoors, you’ll need all doors and windows open, this stuff stinks. Don’t even consider spraying without a good mask.

Consider also it may be illegal to undertake this type of work from a residential home, check your state laws.

If your going to be crawling around under the car, invest in a disposable suit or don some old clobber, the sealer sticks to clothes like you know what to a blanket.

The wheels will need to be removed and cover down your brake rotors, under-sealer over-spray is a pain to clean off.

I have a workshop compressor and under-selling gun, it blasts on the heavy sealer in a thick coat.

Undercoating I Use

Undercoating is undercoating, right? No, I’ve been using Hammerite undercoating for years, this stuff is a little different, it’s got a phosphoric rust inhibitor built-in.

It’s a mix of bitumen undercoating and rust-inhibiting cavity wax. It basically stops rust and repels water, and all my cars are coated with it, I’m a little bias here because I know it works.

Hammerite Waxoyl under-sealer is available in aerosol, so if you don’t have a compressor you can still get a Pro finish. It is possible to brush it on, but the finish won’t be great. Undercoating will need an hour between coats.

Hammerite isn’t available everywhere and I don’t see it on Amazon, but I have also used Rust-Oleum and can say it works great, but it doesn’t have Waxoyl added, you’ll need to buy and spray the Waxoyl separately. Here’s a link to the Rust-Oleum On Amazon.

Undercoating is thick, so I like to heat it up a little, I drop the sealed container into some warm water for few minutes, goes on like butter.

Classic car under-coating with Waxoyl

Conventional under-sealer is made from bitumen and it’s pretty thick, the warmer it is the better it will lay down. It is possible to put too much on, which can cause it to crack, and cracking lets salt and moisture in. When applying, try getting the coats as even as possible.

Under-body sealer


Thin the under-sealer with paint thinner if you find it difficult to work. I like to lay down one thick heavy coat. If you’ve thinned it, you may need two coats.

under-body sealer
Under-body sealer
back axle

If you’ve used an under-selling gun, it will need a good cleaning with paint thinners. I run some pure thinners through the gun until it runs clear.

I’m pretty good at cleaning my kit, but I confess, I killed a perfectly good under-sealing gun. It was late, I was tired, there was beer and stuff……… I’ll carry the guilt always.

That’s it, you’ve done it, tomorrow you can bang the wheels on her, and she’s good for another five years, easy.

Do All Cars Rust Underneath?

Rusty axle

No, not all cars do, the climate and how the car is maintained over the years play a big part.

Consider a car that lives in a salt state, years of driving on salted winter roads causes salt and grit to get behind fenders and into chassis rails, if it isn’t washed down, it will eat your car.

A car that drives on unpaved roads regularly, will likely have a broken under-body sealer and whenever you have broken paint you have corrosion, it is just a matter of time.


Some cars are less susceptible to corrosion than others, the climate is major, if your car lives in a dry state like Texas, it’s much less likely to have corrosion.

Some cars are made from aluminum, the chassis, and the body panels, the Audi A8 and Jaguar XJ6 were made completely from aluminum which doesn’t corrode like a conventional steel-bodied car.

How Do I Stop My Car From Rusting?

Car rust

A classic car or any car is a major investment, for most of us, after our homes, it’s probably the next most expensive and important purchase. It makes perfect sense to protect it, especially the bits you can’t see

I don’t use my classic in winter, our roads get salted so there’s no way I’m taking her out in that. Everyday cars will obviously be used in winter and summer, the underside of your vehicle is the most likely place for corrosion to start.

That’s because, unlike the body panels which are smooth and flowing, the underside is angular and easily catches road salt and dirt. Now add some water and time and it will happily eat your car.

To protect your investment from corrosion, have your vehicle’s underside power-washed, under-body sealer inspected for cracks, and the chassis for corrosion once a year.

Shiny paint wok

If you inspect every year you’ll catch it before any major damage is caused, rust won’t eat your underside away in one year, it’ll take at least a half dozen consecutive years of neglect before serious damage is caused.

To prevent corrosion:

  • Garage your vehicle, especially in winter
  • Wash exterior once a week and under-body once a year
  • Wax the paintwork and chrome once a month
  • Have any water leaks repaired quickly
  • Have broken paintwork repaired quickly
  • Re apply the body undercoating every 5 years or as needed
  • Have the chassis rails, sills and door bottoms pumped with Waxoyl

If you want to take care of your own car, then definitely check out this article “How to care for a classic car”.

Related Questions

What is cavity wax? Cavity wax is a phosphoric rust inhibitor, it’s sprayed into difficult-to-reach cavities such as doors and sills using an under-body sealing gun with flexible pipe or aerosol can. It seals and repels moisture, preventing corrosion.

Is undercoating needed on new cars? Yes, all cars new or old need rustproofing. However, if you live in a dry climate, your car will last longer. Winter road salt, moisture, and debris will damage the thin paintwork on the underside of a new vehicle. The best time to underseal a vehicle is when it’s new, the sealer will lay down better and last years longer.