DJP, had you put any DT on the headstock before you used the nitro spray? If you have, you are going to have endless problems as nitro wont sit properly on an oil and wax based finish. It does need to be bare wood.

If not, then just keep on spraying. The wood will absorb the first couple of spray coats so it's bound to look rather rough to start with. You'd normally use spray highly thinned clear lacquer coats or a sanding sealer (thinned lacquer but normally with a bit of filler) on first before spraying the normal clear coat lacquer on, but do a couple of coats, let it dry for a couple of days, sand it back flat and things should be easier from then on.

If you are doing just the headstock, then one can should be fine. You'd normally expect to use a 2-3 cans to clear coat the entire guitar.