Skim-coat

You can skim-coat the piece with spackling comound, following the length of its grain and dragging the knife over the surface at a slight angle so that you fill in only the grain.

The best way to avoid the problem of photographing is to skim-coat the entire surface after the third coat of joint comound has dried.

Epoxy filler adheres well to itself, so any small areas left from the initial fill can be skim-coated and blended into the repair.