I'm restoring a small travel trailer, and want to paint the tongue and platform for the gas canisters. What are the preparation steps I should use, and what kind of paint would be the most durable?

I like laquer for outdoor use.

If you sand or sandblast to bare metal the metal should be etched. Etching supplies are available at an auto paint supply shop.

Wire brushes can help. If you have difficult rust, you can polymerize it with something like Extend.

Standard body fill materials like Bondo and Bondo Scratch and dent can fill imperfections.

Progressive sanding and a final wet sand with 400 grit paper will give you a nice finish.

See here for some ideas: Have you used Dupont Metal Conditioner 5717S -phosphoric acid - Pelican Parts Technical BBS (http://forums.pelicanparts.com/porsche-911-technical-forum/254447-have-you-used-dupont-metal-conditioner-5717s-phosphoric-acid.html)

Yep, did some major body work on two cars I owned. One when I was a teenager. The other had flex rubber parts like bumpers and laquer.

Repainting rusted metal is simple if you use a rust converter product.

Rust converters chemically alter the rust into an inert material allowing for easy repainting.

All you need do is brush away any loose rust and paint, then apply the rust converter.

You can repaint within minutes.

Here is one such rust converter product from Krylon:

Amazon.com: Outdoor Spaces Rust Converter Spray, RUST CONVRTR SPRAY PAINT: Home… (http://www.amazon.com/Outdoor-Spaces-Converter-Spray-CONVRTR/dp/B001VOEBAO)

You can find rust converter wherever paint is sold.