works fine under water i used it on the 21 project i did but they make an underwater primer for it to use first. best to use the primer on the entire boat. that makes the paint stick forever. I left that 21 in the water for up to 3 weeks and never an issue. it will work without primer too but the primer will make it hold so much better even on the top side. all marine paints say topside paint except bottom coat paint. just use it, the stuff works great.
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