You can only use sandpaper for wood; it is not abrasive enough to use on metal. The grit size on the paper will vary. It depends on what you need to sand. Use smaller grit for smoothing areas. Larger grit can remove larger bumps. Sandpaper will wear easily, and you will often need to replace it.
Use an angle grinder for metal. The grinder is portable and needs electricity to function. It has an abrasive disc that rotates at a high speed that can remove a small or large amount of the surface. Hold It at an angle and move it back and forth in a linear motion, slowly removing the unwanted part of the surface.
The belt sander can grind the surface of wood and metal. It incorporates sandpaper or anything equally abrasive onto a belt that rotates around a set of wheels at a high speed. Hold the material up to the abrasive belt and sand the surface off.
You also can use the disc grinder for wood and metal. It is similar to an angle grinder in that it uses a rotating disc. However, it is attached to a column and is not portable. The disc has an abrasive surface that, when switched on, will spin fast. The diameter of the disc will vary. A table is In front of the disc. Lay your material on it and hold it against the disc.