While I've never seen galvanized springs (galvanizing probably pops off with flexing) you can prevent your springs, frame, wheels, nuts, etc. from rusting by applying a coat of used or cheap automotive oil to everything you don't want to rust. I use waste/used oil and an old hand towel and just slather it on. My trailer is 12 years old and nothing is rusting yet and I do 90% of my fishing in salt water. Each spring I'll bring my oil in a gallon paint can with the rag inside and after fishing when the trailer has had a chance to dry off, I coat the whole thing before loading the boat. It does leave a light brown sheen on it, but that's better than a red/rust coating. Just make sure to bring clean up GoJo and towels. You can also do it right in your driveway, but it's much easier and quicker to do it without the boat on the trailer than all that crawling around on the ground.
