Wax oil, should i use it?

[Chris KC]

Just don't use silicone sealant on your engine!

No seriously, the black brush on stuff is jolly good. My car had nothing else on the underside for 30 odd years (i.e. on bare steel) and it only rusted when the coating started coming away - it does eventually dry out and fall off if not re-coated. I agree with Hedd though, paint then apply over the top. Then take a look underneath once a year or so and address any signs of failure.

Check you don't need to weld anything before you apply it rather than after!

but doesnt wax oil stay supple? i thought the really old stuff that people put on that was like a tar, i thought that eventually dried out, but i thought wax oil stopped.......well,  waxy?

The stuff I had on for the last 30 years was, I think, 'Spectra Brushing Underguard'. It eventually hardens, probably wouldn't have done had I given it an occasional re-coat. I have now applied Hammerite Underbody Seal 'with added waxoyl'. This goes on nice and easy with a brush and gives a neat black finish which I expect to remain flexible for some time. I did my wings and running boards separately some ten years ago now and they still look like new. The spray-on waxoyl is a clear liquid wax, I don't have direct experience of keeping it long term but likewise it will probably benefit from an occasional re-coat.