For anyone builing a trailer, you can have it hot dip galvenized after it's built. Most large cities have a facility, look in the yellow pages. You will have to take it apart and pull off anything you don't want dipped in acid or molton zinc. The yard will charge you by the weight of zinc they put on. My tilt trailer was done that way and shows no rust after 10 years.

I asked my materials people about POR15 for extreme salt environments vs galvenizing. The response was, for long term (10+ years) it is something you might want to put over galvenizing but not instead of galvenizing.

It was interesting, painted galvenizing (the process used for good car fenders) is rated for 1000's of hours exposure to salt spray vs 10's to 100's of hours for most coatings.