Clean it with distilled water, not tap water

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If it's really gross, which seems likely, I would suggest hosing it off outside first, removing any and all socketed IC, cards, plugs etc (carefully noting where they came from) then washing it in the dishwasher (no soap, no drying cycle, and no other dishes or food items), then rinse with distilled water to remove any mineral buildup. A final rinse with denatured alcohol may be a good idea too. Then you should be in a position to get a good look at the condition of the board. Leave it to dry in a warm dry spot for at least three or four days, preferably a week before attempting to power it on - water has a way of sneaking into nooks and crannies where you can't see it, even if the board looks completely dry.