Painting over wallpaper

Idea

Painting over wallpaper can be as easy as painting a regular plastered wall. This happens when the wallpaper is glued to a perfectly flat wall and there are no seam flaws. As this almost never happens, you are likely to come across some of the challenges mentioned below.

Loose Edges
Make sure you glue them all before you start painting. DYI shops sell quick-drying wallpaper pastes for repairing loose edges etc. After putting the paste in the gap between the wall and wallpaper, you need to apply pressure for about 15 seconds. Afterward, the paste becomes adhesive enough so that the wallpaper does not separate from the wall.
Be diligent in finding all the loose edges. If you leave them loose, there is a fair chance that these spots become even larger after the paint has dried.
Use a toothpick, for example, in order to try to gently separate wallpaper edges from the wall. Use another toothpick to get the paste between the wall and the wallpaper.
Consider also water-based sealant to fill in gaps between the top edges of the wallpaper and the ceiling.

Seam flaws
Use dry wall mud to fill in the gaps or flatten the overlaps. Once the mud is dry, sand it smooth with a fine sand paper.

Bumps and dents
A vivid pattern on the wallpaper conceals many bumps and dents. After painting such wallpaper, the bumps and dents become much more “visible”, especially on the walls where the light comes under a very sharp angle. Use the dry wall mud as well to fill in the dents and flatten the bumps.

Loosening wallpaper while painting
It is very probable that during the painting some wallpaper will loosen and bubbles will form. Article How to paint over wallpaper advises to paint with oil-based primer first. Such primer does not impact the adhesiveness of the wallpaper glue and it impregnates the wallpaper. Thus when painting over it with a wall paint afterward, the wallpaper does not loosen anymore. Technically this is very nice solution. However, are you ready to smell white spirit in your house for about a week?
Good alternative is to use a better quality wall paint that can be washed with a wet cloth. The first layer needs to be thinner so that it impregnates the wallpaper while it does not loosen it up so much. The thickness of the second layer depends on the pattern of the wallpaper. If it is vivid, you probably need a thicker layer.

Instructions
Given these challenges and possible solutions above, here are step-by-step instructions, which should help to achieve good results in most cases:

1. Handle loose edges first. Be thorough and glue also edges, which seem to "stick" to the wall.
2. Handle the seam flaws, bumps and dents with dry wall mud and sand smooth with a fine sandpaper.
3. Handle the gaps between the wallpaper and the ceiling using a water-based sealant.
4. Apply the first layer of the wall paint. The layer should be thin so that the wallpaper does not loosen up. If it does, do not panic. It should return to its previous state after the paint has dried up. Use paint that can be washed with a wet cloth because it makes the wallpaper partially water resistant (so the loosening won’t happen when applying the second layer).
5. Check the wall for seam flaws, bumps and dents. Once the wall has a plain color, some of those flaws become more apparent. Fix them using dry wall mud.
6. Apply the second layer of the wall paint.

Limitations and Side Effects

The above method works for flat wallpapers. It does not produce nice results for wallpapers with a texture.

It is also important to realize that if you decide to remove the wallpaper in the next round of remodeling, it will be more difficult to remove than now. The steam from a wallpaper steamer or wallpaper removal solutions will not penetrate through the layer of wall paint sufficiently in order to soften the wallpaper paste. You might end up peeling off the wallpaper with a spattle, for example, and damaging the wall.

So the smartest thing could actually be to avoid the “easy” way of painting the wallpaper. Rather remove the wallpaper and hire someone to plaster the walls.