If the wallpaper is still completely attached to the wall, it is OK to paint directly onto it. If the wallpaper has an embossed pattern, this pattern will show after the application of the paint. In such cases, you may prefer to strip off the wallpaper or to apply wall putty in order to hide the embossed pattern and get a smooth surface. Make a trial run on a smaller surface. If the wallpaper peels when applying the wall putty, you must remove all the wallpaper before applying the wall putty. You can also use Colorex Blocka sealer, a primer that is impermeable to water.