Hey bro, get a bottle half full of dish washing detergent and fill the rest of it up with hydrogen peroxide, get you a good scrub brush and apply the liquid to the board and scrub it.

You could boil some water, right, mix some soap up with it, pour it on the board, and that should pick the dirt up. And then you just hit it with a power washer or keep dousing the grip tape in hot water until it's dead or clean.