Hi Mate, unfortunately there is no easy way to restore the underside of a car, mine has been of the road since last september getting a full restoration of the whole car. The underside of mine in one word was "minging" so after it wasstripped of all the running gear etc, I stripped of all the black underseal using a good old homebase paintstripper/ scraper, then it was fully washed down with parafin to disolve all the little bits of shit in all the grooves, then it recieved 3 coats of stoneguard spray and left to harden for a 48 hours in a heated garage (space heater) it then got 2 coats of self etch primer sprayed onto the stoneguard, 2 coats of mallard green and then, as much clear coat lacquer as we could get on without it running/ dripping......the best way to do this is to have the garage very warm as the spray dries very quickly, my garage was reading 40 degrees celsius on the thermometer, god we were sweating.
And now, i'm finally putting her back together with all my powdered bits n bobs
it is a lot of hard work and even more time and patience that I thought i never had but it is worth it in the end

pictures don't show much of the floor unfortunately.
cheeRS