They are usually Rubber
If they are rubber like mine, then I'll suggest what I use. Since I bought the boat new I have been using that Simonize tire protectant product that gives a wet look to tires to protect it from the sun's UV rays. Hopefully that will extend the life of the rub rail, I use it on all exposed rubber stuff autos included. If it is metal...??? I found another car product that adds a bit of shine to aluminum or magnesium rims to make them appear more chrome like. It works well on my aluminum car tire rims and also really puts a shine on the stantions, cleats and other metal trim of the boat.