I agree with Randy that the oil-based Rustoleum in the non-aerosol variety can be very durable. I painted an air compressor and an old scroll saw with it, and it was tough stuff - the saw was eventually left outside in the elements for years, and it never rusted.

I'm not sure how long it took to fully cure, but I suspect it was several months. For all the prep time involved, I'd sooner take a frame to the powdercoater.