We live near the ocean also, and you have to stay ahead of it the best you can. As mentioned, rinsing with fresh water is essential. The previous trailer had a totally enclosed underbelly, and torsion axles so it was very low to the ground and a real pain to get underneath to keep it up. The current trailer is nice and high, and even though the axles and springs are all exposed, it is easy to get underneath to touch things up. I have been hitting any rusty spots with Rustoleum Reformer which dries to a nice flat black, and things still look great.
http://www.rustoleum.com/CBGProduct.asp?pid=40
When not in use, we keep the trailer covered to try and keep as much salt mist and air off as possible. When we head south in the winter, it gets a good wash as soon as we can - usually try to find one of those coin operated wash bays big enough to handle the trailer. It seems like there are plenty of them in Florida - probably because trailer boating is so popular.
The truck gets it worse as we take it on the beach to fish, and of course it sees more miles on the salty roads. I had a local body shop apply a waxy black undercoating - not the old fashioned rubberized stuff - which seems to be holding up very well. Again, I hose it down often and get underneath once or twice a year to touch it up with a similar product sold by Eastwood
http://www.eastwood.com/rust-solutio...k-13-5-oz.html
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Bill & Kate with Zeke (parti-poodle) & Bailey (std poodle)
Stone Harbor, New Jersey
2014 Forest River Wildcat 272RLX Fifth Wheel
2018 F-250 4x4 Crew Cab 6.2L Gasser