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MG MGB Technical - Cleaning the underside

How should I clean away the oil and muck from underneath the car before protection.

The plan Pressure washer followed by a wire brush down with Simple Green Automotive. Then a coating of Waxoil.

Is this a good method?

J Breslin

Power wash, let it dry, apply a good amount of Simple Green Automotive, let it soak in, scrub with brush, power wash, let it dry, apply a good amount of Simple Green, let it soak in, srub with brush, power wash, let it dry, apply a ....... you get the idea (keep plenty of cold beer about).
I say let it dry after the power wash so the excess water doesn't dillute the Simple Green.

I don't have any experience with Waxoil. I don't know how well it would work on the exposed underbody of the car. The old method was to use the tar like undercoating spray. Most on these threads don't like using it but theory is if a stone or other object hits the underside and chips the undercoat, it would eventually fill in as it doesn't completley dry. I've heard of some folks who use the spray on truck bed lining products such as Rhino Liner. I've heard this hardens well and is difficult to chip, that said it would also be difficult to remove.

Any one know who sells Simple Green Automotive in the UK?

On my roadster I am scraping off the old muck with an old wood chisel. It comes of pretty easily and there have only been a few small bits of surface rust underneath. Then I used a wire brush on the angle grinder over it all. I then clean everything with hot, soapy water and a scotch brite pad and treat any remaining rust I find. Then I put on two good thick coats of Hammerite number one rust beater (this isn't the normal Hammerite stuff though). It is meant to be rust killing and seems to be fairly flexible and it sticks well. I then overcoat the whole lot with spray on black underseal. If you use the proper gun you get a very nice finish. Then finally I will spray the whole thing with a rust inhibiter like Waxoyl so it hopefully soaks into any small areas or holes in the other coatings. Then keep it clean, re-apply the waxoyl occasionally and I think it should hold up really well.

Sounds like a lot of work and it is. Scraping the old stuff off lying under the car on your back is a real bastard of a job. End result is worth it though.

Simon Jansen

I finished my "underside" a couple of years ago. My method endded up being:

Paint stripper on bitumen to loosen. scrape with plastic scraper( cheaper and don't gouge metal) to remove excess bitumen. (the paint stripper just about melts it). Use plastice wrap on the top of the stripper for effect. Clean up the residual with a wire brush on a grinder. The Josco ones balance better and last longer. Then use dexoidine (mild acid) to remove the spider rust and brush any heavy rust off with grinder. Prime, paint, sesl and spray on a coat of fish oil. looks fanatastic.


I painted the underside of my 67 B last year with POR 15. It comes with a cleaner and primer prior to using the final coating. Put the car high in the air on axle stands and start soaping it down. Make certain the axle stands are secure. It took me many hours using different brushes to wash every where before using the POR products. Chose warm weather and let everything dry ou tbefore and painting.

An air compressor helps with drying wet locations.

Andy Preston

I wouldn't use Waxoyl on exposed surfaces. It's designed for cavity protection. On the underside of the car it will pick up tons of grit in warm weather, because it remains sticky, and it will get blasted off. You can buy undersealing compounds containing either Waxoyl or Dinitrol. These are much better in my opinion.

By the way, I cleaned off old hard underseal with a needle attachment on my air chisel. Fantastically effective.

Mike H
Mike Howlett

I applied Waxoyl to the underside of a car more than 20 years ago and I have had no problems whatsoever. I reapply it every other year where it wears off like in the wheel wells. Still no rust and easier to apply than undercoating.

The rubber blocks holding the wires and brake lines will dissolve using paint stripper and I recommend new ones after the cleaning.

Prepare to get very messy and use a full face guard and a air filter and wear clothes that cover almost your entire body like a jumpsuit. I also highly recommend laying plastic on the ground to trap all the loose debris.

The best time to do it is in the spring or fall so you don't sweat to death. A fan placed under the car will help the drying out process.

BTW, Simple Green is not a strong enough detergent. You'll need something stronger.
Mike MaGee

This thread was discussed between 30/06/2005 and 01/07/2005

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