I'd scuff sand and apply a coat of rust inhibitive primer followed by a coat or two of exterior oil base enamel.

 

Sandblast it then use rust-oleum paint.

Rust-Oleum Stops Rust 1 qt. Protective Enamel Flat Black Interior/Exterior Paint 7776502 - The Home Depot

Acquire the Rust-Oleum Stops Rust 32 oz. Black Flat Protective Enamel Paint 7776502, dries in approximately 120 minutes formula helps protect against corrosion and rust at The Home Depot

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As mentioned, sand the rust (you can also use a wire brush), clean well, spray gray primer and then spray rustproof paint.
The Gas Company does this when they replace meters or do routine services.

 

IMO the best "paint" would be cold galvanizing... unless you are wanting to paint it the color of the siding so that it won't stand out. DTM (direct to metal) paint needs no primer.