Ronstan Boatsmart Gelcoat Restorer 500ml
If you have left your boat in the water for a few days and it has a brown stain on the bottom that you can't wash off then Gelcoat Restorer is what you need. It's also great for removing any rust stains. Designed for doing large areas, it is strong hydrochloric acid based product so you need to be careful where you use it and where the run off will wash away to. Make sure you test it first on any coloured sections of the hull and don't use on any metal components.