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Removing mildew stains from boat gelcoat

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So, finally getting around to cleaning the boat after winter storage. The mildew got pretty bad on it this winter. Pressure washing got the majority of the mildew off (boat is an off white/eggshell color), but the mildew has kind of stained the gelcoat. I've tried a couple of different cleaners, but the stain is still there. 


So, what will remove the stain that a 4,000 PSI pressure won't?

Through some research, most are recommending muriatic acid (25% muriatic acid 75% water), spray on, wait a minutes, then rinse off. 

Open to any ideas.



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Have you tried soft scrub?  I used to use that to take the red clay stains off the white gas tank/fenders on my Honda 350 X after a day of muddy riding.  The tank was white paint and the fenders some kind of white plastic.  Made that big Honda shine like a new bike.


While I've not messed with mildew stains I've used a polish called Never Dull (in a silver metal can at the car parts places) to remove rust stains from glass as well as brake dust stains from painted wheels on the Z28.


The only thing I can say about the muriatic acid is I've used it to clean off aluminum.  Do not even get close enough to get a whiff of it.  It will choke you in a heart beat and it takes a couple/three attempts to get what feels like breathable air back in your lungs.  BTDT and I had the rag at arms length, but the breeze blew the wrong way, or something....I won't forget it though.

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Well, i tried some industrial bleach. Sprayed on and scrubbed, let sit for 15-20 minutes...it lightened the stains a little, but they were still very visible. Next was the muriatic acid. I found a small spray bottle in the shed, mixed a couple of table spoons of the acid to about 6-8 ounces of water, and sprayed on the worst of the stains...in the case it had adverse effects. Within 5 minutes, the stains were completely gone. I mean completely. No trace. Rinsed thoroughly with plain water. Boat was back to bright white. The only issue I've seen using the acid is, the spray bottles don't last very long. You have about 15 minutes before it won't spray anymore. 


It got stains out that had been in there for years. Now, I need to get it waxed.


For those worried about the mixing of bleach and acid, I thoroughly rinsed the boat after the bleach before applying the acid. 

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  • 5 weeks later...

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