For reasons I can't fathom, birds (probably sea gulls) have been using my boat for bombing practice of late. Hasn't happened earlier in the season. Each time I go to the boat, I've had to spend nearly two hours trying (only partially successfully) to get rid of hundreds of slpotches of staining purple poop. So far, I've tried a variety of products with only mediocre success. The abrasive liguid 'SofScrub'seems to be the best, but I'm reluctant to use it as often as seems needed. What have others used to clean up this crap?