Classic cars forum & vehicle restoration.

[Rick]

Some swear by it, others swear at it .. have you undersealed your car?

RJ
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[Ashley]

Mine are first sprayed with cavity wax inside all cavities then outside, then they are undersealed with a far thicker, stickier version of cavity was sold as underseal. It's thick and sticky and never hardens and then we spray another coat of cavity wax on top. It's very effective in that cars that originally rusted in ten years are usually rust free after more.

Old fashioned bitumastic type undersells do more harm than good IMO.

[Ellis]

Underseal as I understand it, the bitumen based underseal in "Schutz" type cans is not really suitable as a rust inhibiting or rust preventative measure.
I have found that it dries out quite quickly and can be peeled off like a skin.

In the old Land Rover community around here the trend now is to use chainsaw oil both inside and outside the chassis. I am told that it's "throwing" characteristics enable it to creep and leave a water resistant film.
My own Series 2a is having something of an overhaul including a new bulkhead so I'm going to ask one of the local Land Rover enthusiasts to treat mine to chainsaw oil injection when it is complete.
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Starting Handle Expert

1964 Jaguar Mark 2 3.4 litre
1962 Land Rover Series 2a 88"
2002 BMW M3 E46 Cabriolet

[MikeEdwards]

I'm in the process of a rebuild now, and I've gone for "stonechip" rather than the more traditional underseal. But as it's going to be doing relatively low mileage, I also will be checking it periodically so the traditional issues such as damage to the coating allowing water to get behind it would hopefully be caught quite quickly.

When I put the Firenza back on the road I kept the wheelarches free of stonechip of any kind, just a good coat of paint. The only trouble is that whenever you throw a little stone up into the arch off a tyre, it sounds like someone is throwing rocks at you, so last year I finally relented and added some stonechip.

[baconsdozen]

Best stuff I've ever used has been Eureka fluid film. Warm it up so it flows well and do it every year. I dont rate underseal and Hammerite I've found even worse. Then again after never having seen a rusty front subframe but changed loads of rears on minis and 1100s etc,oil is probably as good as anything.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).

[Bitumen Boy]

I've had good results from bitumen underseals in the past, and suspect a lot of the bad press is down to unrealistic expectations - what other coating would you expect to last 20 - 50 years without any maintenance at all? There are no "one coat" solutions, rustproofing a vehicle - ancient or modern - used on British roads in British weather is an ongoing commitment. I always used to black the sills, wheelarches etc on the Minis and Acclaims, and still black the wheelarches on the Herald, and have found the trick is not to let it dry out. A quick clean and fresh coat of black bitumen paint (which can be dead cheap) over the underseal every year and you're laughing. It needn't be onerous, on the Herald for example I usually do it after checking the brakes, with the wheel already off it only takes a few minutes per side. I did have one very smart Acclaim that came to us with nearly immaculate white paintwork and so initially sprayed the sills over with 4 coats of white stonechip, hoping to preserve its looks, but after a few months of regular use the sills had stonechipped anyway and rust was breaking out so I whizzed those spots back to bright steel and treated them to some red oxide before applying a good thick coat of bitumen underseal, after which it gave no more trouble...

For saying all that, however, there are often better solutions these days. In fact I think the only commonly available bitumen underseal now is the Hammerite version as sold by Halfrauds, and I always considered that one to lack the body necessary to make a sound job. Now I like the wax based fluids - those from Bilt Hamber are good - which are very effective and easy to apply. Anyone who's familiar with the underside of a Herald will know how the separate chassis design tends to create many and varied inaccessible nooks and crannies, meaning that any paint type coating that relies on surface preparation simply isn't practical. The waxes are just as effective underneath a modern car and so get my vote anytime. As for stonechips, the Hammerite variety seems to be pretty useless, but I've had fair results with a "professional" version found in the local factors, it's meant to be sprayed but brushes on quite well if that's too much bother.

[baconsdozen]

I know some people who swear by bitumen based underseal with some oil or grease mixed in so that it never completely dries. My P5b was undersealed when new but after all this time its now brittle and where cracked has let water and damp creep in underneath and actually makes the rusting worse. Spraying it with a creeping type oil based rust treatment slows it down and seems to soften the old coating.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).