Matthew, I like what I see at the SPU website. Their tech sheet says to put the body filler on the wet epoxy primer. I used to be a tech rep for a filler company and the chemists said that was a no-no. Solvents in the primer could cause problems or loss of adhesion between the filler and primer. On Dupont's website the epoxys they recommend under filler specifically say to dry overnite (16hrs) or force dry/bake at 140F for 30 minutes.
Yeah, I can get pretty technical, but i can still speak hotrod. Now days I mostly deal with rusty old bodies and I prefer the etching primers on a lot of that type of work.