Similar conditions here in Ohio, salt trucks suck the life out of any coating.
Our process is to Blast the metal 100% clean of any old coatings, rust, mil scale.
Which not only cleans the part, but provides a nice 'hook or tooth" for the powder to latch it's self to.
then we apply a chemical pretreatment which washes away any dust residue left over from blasting, further etches the metal and applies a coating that seal and protects the metal, then rinse, apply a sealer, then dry.
Once dry we apply the powder in however many stages required by the job.

the process we use is the process the military requires for powder coated part for use in combat. This way it can get a full life of use, and abuse, again up to 2 sticks of TNT (jk)