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.

reduced bedliner undercoating
 

Thanks for posting the link to the video:thumbsup: . I just now got around to viewing it. I wonder how hard it is to color match. Also, how big of tip you might need.
Anyone else think this is something worth trying?

Great Link, Thanks MUCHO!!!
Liked the video. Always helps me to actually SEE what others are talking about.
Id like to color match the under side but unfortunately. My car will be White. First time I get into it and BAM!!! there goes my nice clean undercarriage.