It's mureatic acid. You can also get it at pool stores, but home depot probably has the best price.

Sno-Bol works great just like the Starbrite product but is $1.79 for 24oz. Just spray it on let it sit for a minute or so and hose off, would not use on paint. If your doing gel coat and not paint and boat is on a rack instead of your trailer muriatic acid works well also and is fairly cheap from pool stores- it won't hurt fiberglass or stainless.
And of course there is the Mr Clean Magic Eraser if you want to use a chemical free approach and it works..
Just my 3 cents,
Rick

Whats in Mr Clean Magic Eraser that takes out the stain???

I don't know but that Mr. Clean Magic Eraser does miracles on white upholstery.

I use Orange Blast Wheel cleaner(not their grease cutter). Works like a champ and cheap. Available at Pep Boys and other auto supplies. Spary in on and 5 to 10 seconds its white again. Rinse off. It contains oxalic acid...not all wheel cleaners do. Rinse trailer good when you're done as it will spot aluminum.

I think i am going to try the sno ball first. I have worked with the acid before and its pretty harsh stuff. Thanks for the input guys.

Oxyclean also works great on hull stains also thumbs up for the Mr. Clean magic eraser

On/Off is not mureatic acid, it's oxalic acid. Also, 1 cup in 2 gallons of water is all you'll need to remove stains.

It's hydrochloric acid, the same thing in toilet bowl cleaner in a stiffer concentration. Use it liberally straight outa da jug. Won't harm gel, though it will burn your hands (draws the oils right outa your skin) and anywhere on your face is especially sensative, even to the fumes. It will discolor lesser plastics, bleaches the color out. Great for plugged sinuses. Been using it for 15 years, 6 boats that are slipped...I know what stain removal is about.

Ringo

The stuff everone else listed is too harsh and gel damage will/can occur.

You can buy it from Vyper340. I'll send him a PM.