+3 on epoxy, then filler. SPI is all I have been using, and I love the stuff. And you can sand epoxy primer as much as you want- it sands very nicely. There is a video you might want to watch where certain methods using epoxy are compared. The trick with epoxy first, then filler, is to make sure that you have sealed everything with epoxy again (no bare metal) prior to spraying your hi-build primer. Here's the video (no swearing in this one):
Primer and filler test
