I used K&H "C Guard Water Base" chip & corrosion resistant coating. I think it an Oz product only, but it did the trick. It is really designed to be sprayed on & give that nice textured finish, but I just brushed it on & got great results - no textured finish, but it is quite good at self levelling so it is really hard to see any brush marks at all. Two coats was enough to provide a flexible surface which adds sound-deadening as well.
Regarding bitumen-based underbody sealers - just don't think about it ! The stuff does allow moisture to penetrate (as I have discovered under my '71 Vette - rotted out sections of glass to be replaced), it attracts & holds onto dirt like nothing else you have seen, and is a huge pain to remove later when you figure out that it wasn't a good idea ! I spent the first 2 months of my GT40 ownership removing this stuff from the front bay & engine bay - the previous owner had got as far as roughing in the engine & suspension, then decided on the bitumen paint. I tried every solvent known, & finally found that vast amounts of paint stripper, followed by gallons of turps worked best - absolutely horrible stuff !!
Kind Regards,
Peter D.