Many products I am sure are great. But I've found it's not magic. Waterline stains do improve with application of some oxalic acid based product as already noted in the previous responses.
"The Works" done great for me. $1.00 at Dollar Tree.
http://www.dollartree.com/household...aner/500c501c504p6065/index.pro?method=search
Active ingredient is oxalic acid. Saturate a paper towel with the stuff. Wipe on the topsides on a cool day. Enough to wet out the stained area. But not enough to allow run-down to the waterline stripe and bottom paint. After just a short time, stains gone or much reduced. If improvement seen but still some tannin (brown/yellow looking stains) evident, just do it a second time. See attached picture for a one coat application around my exhaust elbow exhaust port. The second application got to very white no-stain again. And another photo of after one paper towel wipe also along the aft section of my boat (but away from the exhaust elbow through-hull)
For my boat, I've noticed that wiping oxalic acid solution on all of the topside area reduces the time/effort needed to machine compound the gelcoat before waxing. I've thought this because the oxalic acid probably breaks down hard mineral deposits that one can't really see but are nonetheless embedded into the pores of old gelcoat.
"The Works" done great for me. $1.00 at Dollar Tree.
http://www.dollartree.com/household...aner/500c501c504p6065/index.pro?method=search
Active ingredient is oxalic acid. Saturate a paper towel with the stuff. Wipe on the topsides on a cool day. Enough to wet out the stained area. But not enough to allow run-down to the waterline stripe and bottom paint. After just a short time, stains gone or much reduced. If improvement seen but still some tannin (brown/yellow looking stains) evident, just do it a second time. See attached picture for a one coat application around my exhaust elbow exhaust port. The second application got to very white no-stain again. And another photo of after one paper towel wipe also along the aft section of my boat (but away from the exhaust elbow through-hull)
For my boat, I've noticed that wiping oxalic acid solution on all of the topside area reduces the time/effort needed to machine compound the gelcoat before waxing. I've thought this because the oxalic acid probably breaks down hard mineral deposits that one can't really see but are nonetheless embedded into the pores of old gelcoat.
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