Oxalic is cheap
and works well but I use a gel product to try and keep the acid off the anti-fouling bottom paint
. Acids will eat the copper right out of the bottom paint
if not used carefully. This could potentially make the one or two inches above the waterline basically useless at repelling growth if you cleaned your boat in-water then loaded it for cruising. If you do it out of the water without a drip skirt you could ruin a lot more of the paint
The hull of my boat is always waxed and I really don't get to much if any tannin or mineral staining. A good coat of wax can go a long way to minimizing these brown stains. I did however leave my inflatable
in the water for a few weeks and got enough staining to illustrate the process.
This is easy and is safe for gelcoated hulls. While I have used acids on LPU paints I don't suggest it unless you are really, really, really careful.
To clean waterline stains I use MaryKate On & Off Gel. This is a HUGE improvement over the regular non-gel On & Off or basic Oxalic. It's also a lot cheaper and stronger than Davis FSR stain remover but works similarly. There are a lot of products out there that contain acid and will work but the On & Off Gel is one of my favorites. It also has much less "acid" odor
than the regular On & Off. This is a great step to do before waxing a boat and it will whiten up most any old tannin/rust stained gelcoated hull.
Wear rubber gloves, this stuff is acid, and use cheap
to apply. This whole process on my inflatable
takes less than 10 minutes and requires NO rubbing. Simply apply with the chip brush and watch it eat away the stains. Regular oxaic acid, or oxalic acid crystals mixed with warm/hot water will work too, and is slightly cheaper, but it is thinner and can drip and run and can eat the copper out of bottom paint
. The gel stays put for the most part with considerably less dripping. This bottle lasts me about four to five years...