Some epoxies sand better than others. SPI and House of Kolor are some of the ones that are better at sanding with minimal if any loading of the paper. Now they don't sand as easily as a high build primer, but they are much better than most epoxies. I do know that H/K has better build properties than most epoxies, they advertised their KP2-CF as filling 40 grit scratches with no shrinking back. Not that you need anything that coarse.. But for something that may take a few coats of epoxy to cover any imperfections, an epoxy with high build properties may be a better solution to eliminating shrinking in the scratch areas as it is a more stable substrate.
In the collision industry, it's a quick in and out the door in 3 days or less (in most cases) so epoxy is not on their list of go-to products. They are more in line with etch primers, high build, spray and go. The typical epoxy takes a good 24 hours or better to fully cure, so it definitely is not something a body shop would want to use. But for a home hobbyist or restoration shop, the better sealing properties of epoxy in a longer term repair process before final topcoating make the epoxy a better solution all around. After all the body work is complete, a sealer is used to both give a uniform color (especially under a translucent, like red) but also to provide a tie coat that does not have the porosity of the high builds or filler that may have been used beneath. Spray a basecoat over any of these porous coatings and those areas will act as a sponge, and any solvents may have adverse effect on their stability, possibly causing shrinking effect or die back in the finish down the road. Most epoxies can be thinned slightly (<10%) to be used as a sealer with good flowout.
