The more time you spend on surface preparation, the better your finish will be. Here’s a handy guide for preparing your walls for painting.
The trick to removing graffiti is to do it as soon as possible. Most graffiti is done with paint, which is applied wet and solidifies as the paint solvents evaporate. The solubility of paint, therefore, is reduced the longer it has a chance to harden. If you remove the paint right away, you will have a good chance of succeeding.