Plan to wall paper. Need to also make some drywall repairs. Have been told that it is not good to hang W/P on new paint, that it will not adhere and come off. Also plan to knock(scrape) off existing plaster burrs to attain smooth finish for paper, is it best to apply a sealer to these scrapped existing walls before papering. Whats a good sealer product?? TY

Any porous surface to be wallpapered should be prapared first to stop the paste/adhesive from soaking straight into the wall, it's known as sizing.
The simplest method of sizing the walls is to use a diluted mixture of the paste/adhesive to be used for the wallpaper and simply paint it on to the surface. As it dries it seals, and will allow the paper to be adjusted more easily during the hanging.

Do any spackling and sanding first. Then give the walls a coat of SHIELDS/PRIMER SEALER before papering. This is a sizing/sealer in one easy application. If the paint has been on for at least 2 weeks it is cured and is safe to paper over.