Hi Mike,
Gary's suggestion may well work but if it doesn't you could try making stand-off masks from stiff card. The idea is that they are held away from the work by 1/8" or so (you need to experiment on some scrap first. It depends on paint, temp,spray gun, pressure etc) and you then spray up to the edge with the gun at 90 degrees to the work. The greater the distance the more fuzzy will be the edge of the line.
Hope this helps.
Regards,

John.