What I do is; (kinda tough to explain, but I'll try) , using 2 pieces of tape, 1 on each side, allow the tape to conform to the bomb, so that the whole tip is covered. Then holding the bomb tip against my middle finger, x-acto held like a pencil. I then spin the bomb slowly, just enough to cut through the tape.
I prefer to do several drybrushings of cromate yellow, as apposed to spraying and risking "bleeding". This is the method I used for all my A-10- mk 82s.
Hope that all makes enough sence to be useful.