What I use is MaryKate gel.............But if the brown crud is real bad I will Steam Clean it. Most Marinas have steam cleaners and do not charge to much if you have them do it.
Had to look at the Forums name to find out if it had been changed to Ebay..............Low and behold it still reads Lake Ontario United.
Good Luck with your Sale.
Marine Plywood with Resin.................Been using that combo for quite some time now with great results! 50.00 for 4x8x3/4 per sheet.......Resin & Plywood total cost is dependent on how big your boat is.