i recommend flap disc on grinder to do initial knock down work
http://www.harborfreight.com/4-in-60...isc-69628.html
i'm half way planned to pull the same thing you are but use jb kwik weld as it sets and cures faster and grinder for clean up then depending on finished product maybe fiberglass or some kind of bedliner,rubberized coating/plasticoat,or recover with thinner padding and vinyl. not really sure lol