I have just finished doing the undercarriage of my '71, by far the dirtiest part of the entire restoration! As for using marine materials, a co-worker of mine wouldn't suggest it. He says there are different chemicals in that spray which inhibit algae growth, and who knows what else...plus marine materials are always over priced. Here is the product I used for my undercoat. Just be sure to clean the surface with kerosene or some other light distillate, and wipe it off before it dries. Here is what it looks like after being applied: