This is a good question. It is very important to ensure that your wall is smooth and free of any obstacles.
The wall surface should be:
- Free of mildew, grease, or other stains
- No lumps or bumps
If the paint is old or flaking, then preparing the surface and painting it correctly is highly recommended. Old wallpaper and adhesive should be removed by sanding, or adhesive remover and primer should be applied to the clean, smooth wall surface prior to application. Any holes should be filled with a suitable filler and smoothed out.