For as long as I’ve owned the boat, there’s been this horrible awful rust stain about 3 feet tall streaking the hull. I bet we’ve tried a hundred dollars worth of fancy boat products, fiberglass cleaning products, buffing, etc. to get this streak gone. To no avail.
The cause of it is very obviously from a corroded mushroom thru-hull thing where the air conditioner discharges it’s coolant water. 
I’ve been planning on replacing it to keep the stain from getting any worse, but it seemed like a lost cause since the stain was already so ridiculous.
So how did we finally solve this dilemma? The Works. This bottle of magic costs less than $2 at the Wal-Mart and wiped the stain off like it was butter on granite.
Now we’ll just add a little wax and a new thruhull for the discharge and mark it off the list!
The easiest solution is always best one. Everywhere you can use cheaper “not dedicated” products with better success rate than “dedicated” expensive ones. Personaly? I would use pressure washer with chemicals.
Nice, I use that stuff on the green discoloration on the packing nut, and bronze thru- hulls Also, it appears that you are sailing just a little too close to that Beneteau….or maybe your just prepping for the BVI’s đŸ™‚