Dear members, the boat I am looking at buying (1975 M-17) has a mysterious deposit just above the waterline, see the photo at http://www.math.wisc.edu/~andreic/P6070387.JPG Can anyone guess what it can be? The boat spent most of the last few summers (except for the last one) docked on a fresh water lake in Utah. What is very strange is that the deposit appears to exist only *above* the waterline. Do you think it could be some kind of minerals from the water (e.g., iron, calcium)? How hard would it be to remove such a deposit? Or a mold, that grows on humid gelcoat, but not in the water? Could it have damaged the gelcoat? Despite the fact that the boat has been in storage for the last year and a half, I know from sailing pictures of the former owner that the deposit was there at the end of the last summer when the boat was sailed, two years ago (photo was of the boat in the water, sailing, and one could see the deposit at the bow). Thanks, Andrei.