So what do you think you will spray on the body that will shrink less than epoxy? There isn't anything except maybe poly primer, and that is not DTM (Direct-To-Metal). So epoxy it is. Any coating that has solvent or even water in it will shrink as the "vehicle" or thinners that made it thin enough to spray evaporate out. But the big advantage of epoxy is that once this has occurred, it cures and becomes "non-reversible," and won't absorb solvents from later coatings. That and many other properties set it apart from other primers.