Join Date: Aug 2017
Location: North Georgia, USA
Racer-X is offline
Re: Rust on underside
Use a wire brush (wire brush cup on a drill, wire brush wheel on a die grinder) to remove as much of the loss rust as possible first. If there's rust deep in pitting, you might want to apply "rust converter" to chemically treat that rust.
After that, you can use a phospohric acid solution (10% or so) to etch away what's left and prep it for paint.
Rustoleum paint is good.
There's a rust sealing coating called "Rust Seal" from KBS Coatings that's also excellent, but it's expensive. They also sell a kit with cleaner/degreaser (similar to any other cleaner/degreaser on the market), a "Rust Blast" etching solution (which is just a 10%± solution of phosphoric acid, premixed) and the Rust Seal coating.
Another well known rust treatment is POR-15. Used as directed, it's excellent. It's more of a coating/primer and really needs a top coat/seal to complete the job. A lot of people use it as the only treatment/coating, but it's not designed as a weather resistant final finish, it's more of a rust coating primer.