I paint it on with a soft sable brush - it's very diluted ink so it doesn't clog up the board surface and I can continue to scratch and layer on color. I do use an airbrush usually at the end with just black to cover up all of the nicks and scratches the board acquires during my drawing process - I'm really rough on my stuff and I take the boards wherever I go so I can get work time in while I run around doing the crazy mother-of-two shuffle. Your style is very detailed and scratchboard requires that kind of patience.