I thought I`d share this little trick I learned from painting real cars on bondo and filler primers.
What you do is when you have roughed out your filler to where you think it`s pretty straight after using coarse sand paper. The what you do is get a cheap can of flat black spray paint at an autoparts store. Lets say you body is grey or white. Lightly dust the black spray paint on it very lightly, And hold it 12 inches away and spray fast on the surfaces til it is covered.
The when you start sanding again with fine grit sand paper, The light dusting of black will help you find low and high spots, shrinkage and the rougher sanding scratches. keep sanding til the guide coat is gone.
This photo is an example. The car was painted with Nason yellow filler primer, Then when used red in a paint gun and swiped over it quickly. The best Flat black spray paint ever made in a can is S.E.M Trim black, from dupont. It also makes the slickest flat black paint jobs too.
Heres the SEM link. Trim black is meant for painting moulding on cars and stands up to the element well.
This picture of my guitar is painted with the trim black.