UNDERCOATING SUGGESTIONS

Not a direct answer to your question, but if memory serves powerblock tv did a special, one of those help a guy down on his luck restore a car of his dreams events, and they sprayed on a bedliner product while the car was on its wheels in a paint booth. You might find a couple pointers there.

'74 Challenger Part I

Not sure which episode it was on, but here is the link to the first one.