This stuff, Slimy Grimy, is the best boat bottom cleaner I have ever used and I have been boating and cleaning boats for over 35 years. It gets the discoloration off with just a simple spray. You still haw to scrub off the hard deposits, but the green discoloration disappears with a single application. I had stains I had missed over 2 years ago, (I have a very intricate bottom design on my boat, and had hurriedly stored the boat away for 2 years) and they disappeared with a single spray of Slimy Grimy. Buy it, you won’t regret it.