Re: Using a metal trailer in saltwater
If you have to use a plain painted steel trailer in saltwater, it's best to not try to plug any holes, you want it to be as open as possible to make it easier to wash away the salt. Salt will stay in places like frame tubing, clamped or layered steel such as a suspension slide carriage, and it will get under loose paint. The best bet would be an open C channel frame not tubing. Open bottom axles are easier to keep clean but also tend to be thinner and will rust out faster.
I boat 90% of the time in saltwater, even a good galvanized trailer will rust. The springs and wheels go first, then the hardware. Frames with non draining cross members have about a 4 to 5 year lifespan.
Lots of lube, keep it well painted, and remember never, ever neglect to flush and clean every tiny groove every time you use it.
I prefer to launch in freshwater and run to the saltwater, but with today's gas prices, that's not always doable. Even when running in some freshwater, I'll still flush and wash the boat and trailer just to be safe. I also like to keep all moving parts well greased, as well as never seize on any bolts that I may later want to adjust or remove.