[Didymus]

One thing that works for me is the BRUSH type Tamiya Surface Primer. First, I sand the mold line with 800 grit paper, to get rid of any edges. To fill that seam, I brush the thick stuff in the bottom of the bottle onto the mold line. After it dries, I sand it smooth.

Then I spray on another coat of primer and check for tell-tale brush marks, pits and any other problems. It's much easier to see defects on a smooth coat of paint.