[Mike 85]

IMO,You'll find the "surfacers" are the high build and close to polyester style primers.
The "advantage" with poly's or epoxy is shrinkage.
Any "2K primer or surfacer" WILL shrink down and what you thought was a slick sanded surface will show some scratches later on if done too quickly.
Epoxy and Poly's shrink but nothing compaired to 2K pluss they are more solvent resistant so scratch swelling is minimal and usually,what you spray is what you end up with.