A Dremel is like cleaning the floor with a toothbrush. Use an electric drill with a 6" coarse wire brush, then topcoat with Rustoleum Rust Reformer, followed by a coat of fishoil primer, followed by a coat of Rustoleum enamel. The job will last longer than you probably want to own the boat.
Rust reformer would actually "work" even if you didn't do any prep. Minimal prep is all that's needed. The wire brush will get all loose scale off and it'll only take a few minutes to do a whole trailer. A grinder will unnecessarily take off too much metal.