I wrote up a quick little how-to with my POR-15 experience:
http://www.flexistentialist.org/archives/2005/02/09/rust_control_wit.shtml
It has a very good reputation, and is easy to use. You can lay it down right on rust, and then do whatever you want over it, paint, bondo, fiberglass, epoxy, anything. The metal is protected, so you can go nuts on top of that.
I'm going to be tackling a little surface rust on my jack holes soon with the POR-15, and then I'll lay down another layer of rubberized undercoating over the POR-15'd areas.
I wish I had the luxury of not driving all winter, or owning a fleet of cars in a big garage, but community outreach education is not a very high income career choice.
In the meantime, regular cleaning and quick treatment of problem areas with POR-15 is the name of the game.
peace,
sam