Need to paint in a hurry and don't want to remove wallpaper? Not a problem. You can paint over the top of most wallpapers without too much trouble, but you'd better have a really good primer. Dark colored wallpaper is harder to cover than lighter shades of paper, of course, but much depends on the color of the paint that you will be using and the type of wallpaper that is hanging.
Don't spare money when it comes to the primer you choose for painting over wallpaper. Most primers work okay, but some require you to put coat after coat on before the wallpaper is completely covered. One or two coats of primer should work for most light colors, but dark wallpaper could require three coats. Primer is usually purchased in a shade of white but tint can be added to cover certain colors of paper more easily.
So, if you're painting over light shades of wallpaper with a light shade of paint, tint the primer to a color near the paint color and primer the walls with two coats of primer, letting each coat dry well. If you're painting over light wallpaper with a dark color of paint, it's probably not absolutely necessary to have the primer, but a coat of dark-tinted primer can be applied anyway, for better paint application. But, for painting over dark wallpaper with a light shade of paint, tint the primer to a shade close to the paint, then primer the amount of times needed to completely cover the dark paint. If the room is dim, you could find yourself surprised to later notice, when the room is bright, that the dark wallpaper still shows through the light-colored paint.
Certain wallpaper is difficult to paint over properly, since some paper has ridges, velvet inserts or raised areas. These types of wallpaper should be removed before painting the room, but if that simply isn't possible, try sanding the wallpaper well in raised areas and embossed places. Don't expect terrific results when sanding wallpaper with raised areas, though. And, it's not a good idea to paint over the top of borders, either, since you will be able to see the outline of the border, no matter how many coats are applied. Lightly sanding the edge of the border helps somewhat, but usually a thin line can still be seen through the new paint. Plastic-coated wallpaper or foil-embossed paper should be completely removed before painting. It's nearly impossible to sand all of the plastic coating or foil off and would be very time consuming if tried. The plastic and foil deter paint from sticking and even with primer, results aren't long-lasting.
Fabric-type wallpaper can be easily painted over, but will give your wall a look of texture and not a smooth finish. Definitely use at least two coats of primer over the top of fabric wallpaper before painting. Fabric wallpaper tends to absorb the primer or paint, so extra coats may be necessary for full coverage.
Most other wallpapers are paintable, but remember to first take down borders, buy top quality primer, and apply several thin coats rather than one or two thick coats of paint.