I guess I've been on a quest of sorts to limit the chemistry in the film between the bare metal and the color package. I retain left over paint and primer materials from my spray gun liner cups and observe volume retention or shrinkage. It seems like the spray poly products and 1:1 epoxies shrink less than anything I have tried so far. I guess I liked the idea of a poly film to compliment any poly fillers and glaze work I've got on the body. I suppose that's not entirely true if the spray poly is a hybrid though. It just bothers me having so many layers between my metal and my top coat package. I'll have epoxy, poly, 2k urethane and a sealer... and it bugs me. I guess what bugs me the most is some of the 2K's I've tried which seem so loaded with talc and clays, it doesn't seem like a very strong film. So that's what has me searching, I want a strong, low shrink surfacing film for leveling my panels.
I have not tried surfacing with spi epoxy.
I do typically use a spray on surfacing product on all my jobs, which seems to take 2-3 separate application/blocking sessions before I am happy. I can see the SPI epoxy making me pretty happy in my efforts, I just need to try it.
I have a full spray on a 69 mach1 coming up next. I will try using the SPI epoxy for my entire film package from bare metal on up to a reduced epoxy sealer prior to color/clear on the mach1 mustang.