Thanks to my buddy Joe, I found some 3M rubberized undercoating at FinishMaster on Broadway in Santa Monica. The label is different from the product I used under the car, the cans are 17 oz, and it sprays and dries differently. I’m fairly certain it’s an alternate 3M formula, but the guys at FinishMaster told me it’s the only undercoat they’ve sold for several years, so I bought two test cans.
I masked off and sprayed the rear trunk, which looked pretty good except for a couple wrinkly patches that showed up the next day. When I peeled off the rubbery skin, it took the top thin layer of yellow paint with it. I suspect poor adhesion between the base coat layers as the cause. Luckily, it was only a couple areas where color-sanding residue may have pooled up, and not the entire trunk bed.
As you can see from the pictures, I peeled away all the bad patches and used a (3M) Scotch-Brite pad to buff them out, and then reapplied the undercoat. It dried nicely, matching the rest of the trunk. If I am going to continue with the front trunk and floor pans, I can’t rule out a similar patch job. But weighed against the weatherproofing and sound dampening benefits, I think it’s worth forging ahead. The rocker steps look wrinkle free. Hopefully, the rear trunk is the worst example.