Made by 3M: It's called "Marine Premium Filler" It's got what 3M says a "Vynl Ester" It doesn't shrink as much as most of the other brands. Easy to work with. They use it on boats and can be used below the water line.
It's EXPENSIVE. I was told about it from a Sterling paint rep. I found the stuff at "West Marine".... a boating supply retailer.