A hot rod builder near me (Charlotte, NC) uses a kit that's used for spray-on truck bedliners. He uses a clear kit and then tints it to match the paint. He also thins it out a lot so that it still has a little texture but doesn't have that obvious "bedliner look." I videotaped him spraying the underbody of a '60s Firebird and thought his work turned out really well. If you are interested it seeing it, the video is short and posted at
StreetMuscleAction.com.