I use muriatic acid. Its in a gallon bottle called "The Must for Rust". I buy it at Double W off of Ridge Rd. It's about $10 for a gallon. I put it in a pump sprayer and stay it on and watch the scum line disappear instantly then spray it off with water. It says on the bottle it does not mess up the gel coat. Stop by there and talk to Chase or Charlie and they'll hook you up.
Be careful not to get it on the trailer, I usually place a tarp over the channels and do it in the grass, not on concrete.
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