First I would determine if any of the components on the board are sensitive to being submerged in water. This would be things like transformers, vented relays, and the like. If it can be submerged, I usually start by washing the board in the dishwasher. If it can't be submerged, I would probably spot clean with isopropyl alcohol and a toothbrush. No matter which method you use, I would then blow dry the board with compressed air, and let dry thoroughly in a warm place before powering.