I do a lot of spraying mostly with a (High Volume Low Pressure) HLVP sprayer.
Much better than rattle cans. But have not found a solution getting inside of thick fret work.
The best way I found was to thin down my solution and using a very small paint brush.
Using the bristles and apply my solution (lacquer). I try to "stuff" the bristles in the cuts. I then do a top coat with my spray gun
Not perfect but better than just spray.
Of course, do this on both sides, starting with the back.