Honestly you don't really want to put any wax on there. It can get sticky. It's used in skateboarding because it's a dry sport. Also on ledges it smooths out any bumps while adding a sort of lubrication.
Skiing you have water which is a big help. Rails should never need any wax. You can get some silicone spray and that will help but usually would only use it on sticky boxes.
What is the surface of your rail like? If it isn't smooth or has rust you might want to flap or wire wheel it. IF you have an angle grinder, or borrow one, or buy a ryobi at home depot for 29 bucks or a chi electric at harbor freight for 15 ish it's a good tool to have. Get a wire wheel, not too heavy that the weight will ruin the grinder. That wire brush will take off the rust and old paint and clear it down to bare metal. If there are knobby sections or it's really bad a flap disc is great. Unless it's totally shot I would stay away from a 36 grit completely. If there's a bur that won't come out hit one section if needed, but don't do the whole rail like that. You're going to want 80 or 120 but it's fine to use something more coarse if it's beat and then come back again to smooth it. Also flap discs need to wear in a little bit and there's a period where they hit their prime, and then a period where they go to shit. Also keeping it moving to keep from flattening out ad knobbing sections and keeping it in the right direction that you don't have grooved perpendicular to the direction you slide the rail. A little water should be enough if the metal is clean but if not grab some spray. Also if there's old paint, that shit can be impressively sticky sometimes.