Jerry,
You can get a product called RUSTAID from Home Depot that will take most stains off. You can just spray it on with a pumt-up lawn sprayer, let it stay on for about 5 min. then rinse it with water, no more stains.

Good luck,
Dail

FSR Fiberglass Stain Remover. I get it through Overton's. Best stuff on waterline stains I've tried to date. Wipe on, let sit a few minutes and rinse off.

I do my hull with FSR when it starts to look stained from going through dirty channels.............
The FSR is easy on the stainless hardware.

I use a cocktail of 1 part Muriatic acid, 2 part Super Clean, and 6 water for regular cleaning when pulling out. I only use this on the water line. For general cleaning of the hull sides have a separate bottle with 6 to 1 Super Clean. The first bottle is not bad to smell, the Muriatic acid just gives it a little kick.

Okay, one of us is gonna cause an explosion with all of these home made mixes !!

Product called: Zing ........which has hydrochloric acid in it. Used it on my old Fountain. However, since it is an acid derivative, you will need to immediately wax the area you use this on, or the area will become more yellow. InXS

I do have the recipe for aluminum foil, Borax, acid, well never mind. Kids and I sent up one of those weather balloons with acetylene and used a flare gun to shoot at it, and well, never mind... And one time I mixed diesel, with charcoal and gas and put it in ground hog hole with a smoke bomb for a fuse, you get the idea.

Oh, the potato guns with ether is fun. And Tranny fluid and kerosene in a can with an ice cube or two...

On the other hand my hull stay shiny.

ground charcoal, sulfer & potassium nitrate is fun too

That will definitely leave a stain on the boat though....if there's a boat left

I need to make me another tater gun. I gave the last one away.

Thanks for all the replies guys!

Well, 120 mile round trip to West Marine for some On-Off. Had FSR, but salesguy talked me into the On-Off.

It did nothing also! Then also tried laundry bleach, Bleach White, .......notta.
So back to the full strength Simple Green that at least lightened it.
Soaked an area then used a soft scrub brush and followed with 3000 grit paper soaked with the Simple Green, It's Gone!! $60 worth of specialty cleaners and the cheap stuff is what works.
Not sure what the yellow was that the harsh chemicals wouldn't touch, but it's off. Going to hit it with the buffer then a couple good coats of marine wax.

Tim, I have gotten this before. Each time was from our trip to the river when we stay overnite in Admirals Anchor Marina for the boat burning. I hear the boats can just pump out their holding tanks right into the river, maybe the yellow is just plain ol' piss?? I think since the boat went into the shop a couple days after the river trip and was there for over 4 months, it set in more than before when Sno Bol would take it right off.