As a Machinist/welder/fabber..trailers are easy fun projects. I got a salt rusted out trailer here for free. Mostly solid, except the undercarriage/Truck. Then bought a 15 foot Flatbotton of the great "Doc FrankenMerc" Well, redid the trailer, did the boat..(bare hull, but solid), while Thom built me a crank up fresh Merc. So the original free trailer? Ended up $3500 bucks fun! Some pix
Just an alternative idea Mike, but I have used a wire brush (on a drill) to clean them up. Inspect for cracks, broken leaves, etc., then paint them black with Rustoleum. I wouldn't pull them apart, but there is usually a center bolt that holds the leaves together - be very CAREFULL if/when you unbolt they can come apart with force.
I wouldn't sand blast while they're still together, you'll never get the sand out from between the leaves.
Any hint of a problem and I'd go with the replacements, but many times a little cosmetic refreshing is all they need IMHO.
Give a real close inspection and you might just see need to replace them anyway. I have disassembled many and painted some but not on boat trailers. Mostly cars and trucks. I never re-did them unless there was a need to such cracked or broken ones. But since we are talking about old boats, why spend hundreds and hundreds of hours on a nice classic and put it on a busted ,rusty trailer? Just my thoughts. Lets make sure it's safe.
Its so easy and cheap to do new. If ya going perfect orig resto, OK, to work the springs. But if do new, always can sell ebay, Origs" others. Nothing special about springs. But what not said is use, trailer. Going short rides a show, or many miles towing with a vintage on top and dunk it anyway. Salt, our fresh water? Way to many variables here. Simple do what "floats yer boat" !