I have replacement fiberglass rails from Slane. Very sturdy and heavy. We had a temporary PVC rail on the boat until I got them mounted, which was fine.
The original rails were mahogany. They were fine when new, but over the years they get saturated with water and rot. You could have a set made up in wood, which would last you another forty years if they were dipped in epoxy and painted with 2-part paint.
When you take the old ones off, you have to cut the fasteners, if there is anything left of them. The back part of the fastener will fall into the outboard bilge and you can get them out with a big shop vacuum and a long extension. There is no way to through-bolt them, at least that we could figure out, because (I think) they were put on the boat before the fuel tanks and deck were put in. so you can't get to the back side of that part of the hull.
We used a combination of sanding the paint off in that area, 5200, and self-tapping SS screws to secure them to my boat. So far, so good. I painted them with AG by hand and when that cured, put the stainless rub strip back on.
As far as keeping spray down, I don't think they do much. I think their real purpose is as quarter-fenders- to keep damaging objects away from the back nine or ten feet of the hull side, which they seem to do admirably.