If you can get it really clean in all the nooks and crannies underneath, then let it dry thoroughly on a hot day, you can spray the underside, fenderwells, and all the tight places with raw linseed oil. Stays tacky and flexible, and won't damage wiring insulation or rubber parts. My neighbor's 1964, yes, sixty-four, station wagon had this as the only undercoating and has never rusted. He's still driving it, with several hundred thousand miles on midwestern winter-salted roads. Just renew the areas where you have done repairs and broken the skin from time to time.
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We have met the enemy and they is us. [Pogo] S70 T5 cop car : Rough Rider suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps