Copper water pipes may need to be sanded before they can soldered or otherwise repaired. Sanding copper is going to require different methods than sanding other types of material. There are a few ways you can do this successfully.
This is the easiest and safest way to sand a copper pipe. You will need to find a sanding roll at any home improvement store. This is simply sand paper that is on a roll. You will want to use a grit that is designed for soft metal. This is generally made of emery or aluminum oxide grits. You can use what you need off the roll to rough up the copper to prepare it for soldering or sweating.
A Dremel can also be used to sand copper pipe. You can find special discs for the Dremel specifically for copper pipe repair. The speed of the tool should be set to a low setting to avoid grinding through the pipe entirely. Also use care since using a Dremel will create friction that can cause heat. It's a good idea to use gloves and make sure you aren't around anything flammable.