What would be the best thing to put on surface rust on some parts of the frame?

Thanks

There are quite a few options. Easy would be sand it or wire wheel it, then use rust converter in a spray can, then primer, then paint, something like black rustoleum would be fine.
A good option is POR15, that involves applying "marine clean", "prep and ready" then you brush POR 15 paint on.
http://www.por15.com/SUPER-...KIT--Black_p_11.html

If you have a die grinder with a wire brush head, that is the best way to go at it, unless you have a body off restoration in mind.

You have to degrease the area first, then wire brush. The best thing over previous rusted frame is a coating of Rustoleum or Tremclad. I use yellow because that is what is used on commercial machinery and it shows up every little crevice or pit in the metal. Once you have the yellow coat on, then overcoat it with the color you want. I find that a good quality petroleum based undercoating spray is the best thing, but then again, I am in Canada and the car sees salt on the roads.

In Florida you might do well with POR15 but like the man says, the metal prep for that product is pretty exacting. I think in Florida I'd stick with Rustoleum

Arn