Painting - Painting wallpaper

Is it possible to paint over wallpaper? If so should I thin the paint or mix it with anything? What works best?

You can. It is really better in the long run to remove the paper.

Painting over vinyl or flocked wallpaper will be a big challenge to have it last; or be satisfactory, in the case of the flocked paper. I would not attempt to paint over vinyl paper.

Otherwise, the regular paper needs to be securely adhering to the wall, be clean, and dry. Prime the wallpaper with a primer sealer, such as: Zinsser 123; to help hide the colors and prepare the surface for paint. Then paint with your choice of colors of top quality paint. Recall that paint won't repair defects in the wallpapers surface, only change the color.

Can you do it? Yes, you can.
Will it look good? No, it won't.
Does it look clean? No, it doesn't.
Will you be happy with the results? NO!!!!

When I moved into my house last year, I spent 4 weeks (after work and weekends) before moving just getting the multiple layers of painted walpaper off the wall in EVERY room of a small ranch (except the bathroom).

The difference it made is astonishing. Before, it reminded me of the cheap apartments I lived in when in college. Now, it looks like the wall were just put up.

Remove the wallpaper first, no matter what it takes. You WON'T be sorry you did.

My advice is no. There is a good post on this subject in the wallpaper section you may want to read located here:

http://forum.doityourself.com/showthread.php?s=&threadid=116896