I've used this on My Evo and 309 Gti underneath:

http://www.rust.co.uk/epoxy-mastic.cfm

Take a look at some of the stuff they use it on in there gallery.
Also comes highly recommended from classic car restorers.

If your doin a full underside job I would get some Dinitrol cavity wax off that same site above,much bettter than waxoyl for penetrating in box sections etc,alot easier to use aswell.