Primers?

[68protouring454]

that is only when using cheaper products, and or you rush topcoating it,it mostly occurs using urethane primer that is either cheap and or you top coat to soon,it needs to shrink (dry) completely before top coating,if not you will see bondo areas etc where the primer shrank into the sanding marks, dp90 should not do that as it is more of a weather barrier than any thing, its the highbuild urethane primers mostly,you should let the cheaper ones set at least a week in above 60 degree weather, (hardners do not work below 58-60,even the best primers should sit a day or be baked.
the reason only the bondo spots show is because you sanded the area with 80 grit and finished with 180 then primed where as all other places should be sanded out to 400-600 grit,but even though you sanded the primer over the bodywork with400-600,it still has those deep grooves that it shrinks into, very important to let the primer cure.
jake