[konidias]

All you really need to do is just take the airbrush and set the color to pure white, and then you adjust the transparency of the brush to a really low setting, and start going back and forth over a black area. You just keep applying pressure to the area you want the whitest.

To make it even less brushed looking, you can use the smear/smudge/whatever tool to sort of mush it around the way you want it.