Dr. Zooch Saturn V Build Step 2A Primer

For the past year I've been using Dupli-Color Automotive Filler Primer.

I've read about other modelers doing three light coats of primer sanding between coats.
Lately I've done only one thick coat after filling the body tube seams with CWF.
Any remaining recesses in the seams are filled with this filler primer.

This primer sands off in a fine gray dust. It'll still load up your sandpaper.
After that single thick coat, the bulk of the primer/filler is removed with 320 grit.
Sand with the 320 (or some fine 220, the 220 roughness varies quite a bit) until you can just see the white tube starting to show.
Then switch over to 400 grit and smooth it out.

Here's the dry assembled body parts ready for the white undercoat.