Sliding and grinding on unwaxed curbs is not always that easy, and sometimes impossible. Wax is used to fill up the holes, cracks, and so the curb will have less friction overall.
First of all, get some wax! I recommend using paraffin wax(which you can buy at the supermarket, near the spice section), or a regular white candle. You can even use crayons, or soap (or whatever has a waxy feel to it).
Take your wax and rub it on the curb so that you have an even coat all around. Make sure you use a lot of wax, so you probably won't have to wax that curb again for several months after. There are 3 main parts of the curb that you must wax to have a good curb. The top(for slides), the corner(or "edge" used for grinds), and the side (so your wheels slide too). If the curb is really rough, or has lots of cracks, take a lighter, melt or soften the wax, and get it in there!
If you wanna wax a rail, make sure not to "overwax" it, or you'll be on your ass all day because it's so slippery(if it feels like a condom, then you went to far!). Use a very thin coat of wax, and make sure to apply it evenly.
After you wax your curb or rail, it might be a little crappy at first, but give it some time (maybe even a few weeks) for the wax to harden up and turn nice and black.
If you have any questions or comments on waxing stuff, then email me at: [email protected]