Metal polish isn't going to do anything for you. If you've got a rusting steel door then whatever has been keeping it from rusting for most of its life (galvanizing, paint) is wearing off. It's not feasible to re-galvanize your old door, which leaves you with paint. You'll have to clean off all of the rust before painting, which might be more work than it's worth.

You would treat the steel door like any other steel product. Sand off as much of the rust as you can. Apply a rust reformer product, prime and paint.
Rustoleum should have the whole lineup of products.
Ron

But won't the paint cover the holes in the screen or at least some of them and leave it looking uneven-not uniformly painted? If the door frame itself is in good condition I would see about getting new screen put on it.

"Sand off as much of the rust as you can. Apply a rust reformer product, prime and paint."

Only loose rust should be removed before using a conversion coating.
The coating needs rust to react and bond with.
A lite pass with a wore brush to remove loose material, then appply the coating (auto paint stores are a good source of quarts of the solution).
The coating react with the rust to form iron phosphate, a stable black coating.
You can then paint over the coating.
Avoid using water based (latex) paints as they can create additional rust.
Krylon type spray paint works well, or even auto body spray paint.

Unless the door is an non-stock size, or you've got considerably more time than money, I'd price a replacement for DIY installation before investing the time in refinishing.

Quality finishing materials are no longer cheap.