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Random Quote: Good work ain't cheap and cheap work ain't good.
OK, I've searched many posts and have tried barkeepers friend, the works, gel toilet bowl cleaners. Some (mainly the gel bowl cleaner) lightened the dark spots, but they are not all gone. I don't know if I'm just looking harder than everyone else or what. Now this isn't the regular scum from leaving it in for a few days. It is from way back in a stagnant freshwater canal. The darn water looked like black coffee. I should not have gone but it was too late to turn back anyway. I think it may be tannins from plants? Can these deep stains be taken out?
Thanks
if it is stained so deep that toilet bowl cleaner can't take it off, it's probaly not coming off...might need to sand with 1500 grit paper and re-compound it out....
That's what I figured. It probably won't come off, but really isn't that noticeable. I'll try the on/off. BTW, it is on an '07 240 SH, so the fiberglass is smooth and waxed (white). I'm surprised I can't get it off.
You might also try a fine polishing compound. I've had pretty good luck with 3M Marine Fiberglass Cleaner and Wax (for lightly oxidized finishes) when nothing else has taken a mark or stain off my hull. It is an abrasive, so easy does it and follow the directions. I wouldn't use it as a first attempt to remove a stain, but it sounds like you have tried other options without success.
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*Sometimes I sound like I know more than I do. Other times I don't.
Mark. I also have a seahunt[225WA] Mine was in the water for a month durring the Rock trophy season. It was pretty green. Sprayed on ON & OFF, let it sit 30 sec. rinsed it off, just like new. Cover you trailer when using it. george
Toilet bowl cleaner needs to have hydrochloric acid to work well. For example, Clorox Gel barely lightened the stain, while Lime Away Toilet Cleaner worked great.
"Oxalic acid". The main ingredient in Starbrite Hull Cleaner. Also the main ingredient in Dekswood deck cleaner at your local home center. Follow the instructions including skin and eye protection. Used as directed it won't harm gelcoat or fiberglass but it will stain a galvanized trailer if not tinsed off quickly. No damage, just a stain. If a couple applications of this product don't get rid of the stain, sanding or ignoring the stain are your only options.
Sno Bowl works well for light stains, but if they are really set in, use the "On & Off" mentioned above, wipe on dry hull, let sit for about a minute, then rinse off.
Here is a link to some before/after pictures of my old boat that I posted on another forum last week.....
If you like spending $$$$, go to the marine store & buy something like ON OFF or anything that has 'boat' or 'marine' on the label.
If you are happy with something that simply works go to your home center & get a nice cheap gallon of muriatic acid. Dilute it by more than 50% (makes it even cheaper). Apply it to hull using small paint roller or pad. Instant reaction. Thoroughly rinse off. No scrubbing.
It will also remove rust stains from around deck hardware.
Thank yall very much. I looked at "the works" bottle that I have and it has oxalic acid in it. I may try some of the others also. Like I said, it takes the tint out, there are just some embedded spots but I'm just going to not let it bother me. I plan on doing the whole hull before I put it up for winter.
Thanks again
Water Stains on Hull 
