Painting Trailer Frame?

We take the campers to a local beach campground very often. When you count up all the weeks, they probably sit there for a total of about 3 months getting bombarded with salt air and perhaps blowing sand. We always hose the exteriors off when we get home.

So I was wondering about painting the frame to help ward off rust. There's already some surface rust in some places.

Any tips for this project? We'll be washing, winterizing and putting the campers in hibernation in the next week or two.