I picked up a can on FSR at WM last week even though the label did not specify that it is biodegradable or safe to use in the water. Like most users, I was going to apply it around the waterline to eliminate the scum stain.
Before I did I called Davis tech support and asked if FSR is safe to use that way. They pulled up the MSD sheet and read it to me. When I heard, "do not allow runoff to enter lakes, streams" etc., that was all I needed to hear.
Took the product back for a refund.
Bayfield 29 - s/v Puffin
'Real Lemon' lemon juice full strength on the stain and let it sit for a few minutes, the stain will come right off. I thought it was lame when I heard it but I works great and you don't have to feel as if you are adding a toxic soup to the water you live on.
I used Barkeeper's friend. I just deleted the before and after pics but it worked like a charm. Didn't hurt the finish any but my hull is old and needs repainting at some point anyway. It's ok but doesn't have that new shine. I'm sure I'd be more hesitant if it did. Anyway, worked awesome. I learned to wipe down a large section then go back after it had set for a few minutes to scrub it off.
The citric acid in lemon juice acts as a chelating agent and attachs itself to iron (rust stains) and can then be washed away. Doesn't work on all stains though. A soft abrasive like Barkeepers Friend will physically remove just about anything that hasn't deeply penetrated the gel coat.
The oxalic acid in FSR is a more effective chelating agent than the citric acid in lemon juice. I can't imagine that small amounts, such as you'd use on a boat, would be damaging to the environment.
I use FSR when I have tough stains, but read the warnings on the label and check out the price. You can make lemonade from 'Real Lemon'. Lemon juice is a effective, cheap, first try on ICW stain. I put it in a hand spray bottle and brush it off with sea water. Thick viscus FSR takes a pretty good amount to achieve the same results.
I do use FSR when I have to and agree w/ you that it is not too bad for the environment. On my Grouper or Hog Fish filet I prefer a little lemon juice. Rich