I can get a few tins of this stuff from work, just wondering if it would be any good for sealing the underside of my bus. http://pages.ebay.com/link/?nav=item.view&alt=web&id=261828820788
Personally I would just clean the underside, repair any rot, paint & coat in a clear underbody wax so I can see any problems starting rather than hide them under a rubberised film to develop in to an expensive mess.
find out if it's cross-linked: ie the surface exposed to air is impermeable.....some tanking paints/slurries are designed as an undercoat, so they prevent moisture ingress from the bonded (wall) side, but remain porous on the exposed side to allow another topcoat to bond to it.... best bet is to nick a bit at work and experiment on a bit of old steel...it might be brilliant stuff there'll be a technical document available from the manufacturer online
having read thru the doc, for steel, it looks like you'd apply an epoxy primer beforehand...this in itself is a good protector for the underside of a bus, but can be made more resilient with an enamel topcoat or the stuff you're looking at...be warned....it's a big job breaking out to do properly
If done properly it could be a good solution. I've wondered about using a Polysiloxane Paint Coating as we used them on lampposts at a previous company I worked at and they were marvellous, but as said above the work involved could be prohibitive