undercarpet

How about a wirewheel on a drill, clean everything real good, make sure that it's nice and dry then spray with a good (cheap) primer. That should help protect it for a while.
The Por 15 we used was the stuff in the small can I just had my girlfriend brush it on the floor, inside firewall, and any other spots that needed it. She used a 99 cent 1 1/5 inch brush and it worked great.
http://www.por-outlet.com/shop/index.php/cPath/24?osCsid=7358a53bf67489efb412aec121eb88ea

We got a pint of it and did the floor, inside of the firewall, inside of the roof, and spots in the trunk and I still have a little left.