Testing stain removers.
Ok, I lied. I did not test all the suggested stain removers. At $10. a crack I stopped after two. Actually SnoBol toilet cleaner is not that expensive. But Behr's Deck Cleaner(Home Depot), FSR, and MaryKate ON/OFF are at least $10.The picture is a before/after. The after is where I used Behr's. I mixed it 1:4 in a bucket and just kept wetting the area with a sponge. After about ten minutes I wiped it with a sponge with fresh water. No scrubbing but several rinsings to make sure I got it off the gelcoat. I did not have access to a hose.I have another picture where I used SnoBol above the line. I could not tell the difference. Same drill, keep it wet for a few minites and then rinse well. No scrubbing. These are fairly powerful acids, one hydrochloric and one oxalic(SnoBol). Try some in a gallon of water and then add more until you get the result you need. Then rinse and rerinse.