Bob, I have never run into the problem you have described but my instinct tells me, as the surface dries out, small pieces of primer material that were worked loose by the sandpaper are being pushed to the surface (as the water dries I mean). If it was my airplane I would next try spraying a coat of primer (may not be practical if you have pulled the tape off the panel lines) and DRY sanding with 600. You may want to follow up with steel wool and a tack cloth.

As far as paint goes, you may want to consider the Stits enamel from F&M Enterprises. I hope this helps and you will let us know how it turns out. I also hope some other folks more experienced than myself will chime in!