how do you paint over walpaper?

The important thing to remember when painting over wallpaper is to prime with an oil or shellac based primer, rather than water based. The glue holding that wallpaper to the wall is water soluble. Using a water based primer or paint directly to the wallpaper risks having the wallpaper let go from the wall.

Before priming, it is a good idea to look for loose corners and high seams in the wallpaper. Cut out loose corners and sand down high seams. This is best done with the touch of the finger tips, as such imperfections are not always readily seen. After priming, the imperfections may be patched with patching compound or spackle and then spot primed.

Finally, the wall may be then painted with a a water based paint, the oil primer now acting as a vapor barrier so that moisture does not get to the wallpaper paste.

Hope this helps

DJS2013,

I am not sure what the nature of the allergen is, particles or vapors. New drywall overlay would certainly stop any particulates. If the allergen is in the form of a chemical vapor, I believe a BIN shellac based pigmented primer would help. BIN is the best sealer against odors such as cat urine. I believe it should be helpful in stopping permeable allegens.

I might note that shellac is a centuries old, time tested coating made from an insect which grows in the Orient, dissolved in grain alcohol. It is so safe, that it is used to coat medicinal pills.

Another possible impermiable barrier would be 6 mil plastic placed directly in back of the drywall before it goes up.

For reasons which I won't elaborate on here, impermiable barriers on interior walls are a bad idea in hot , humid climates where air-conditioning is used.

Hope this has helped somewhat.