Q. I plan to wallpaper the ceiling in my bathroom where the old paint is flaking. Sanding and three coats of undersealer have not stopped the flaking. Can I safely wallpaper over this kind of surface? Jo Goodwin Rancho Palos Verdes, Calif.
A. You have two choices, either of which should work well enough to hold the paper on the wall:
* Scrape and sand the wall so as to remove all the old paint. No sealer will hold the paint down well enough to hang paper over it.
* If the peeling is so bad and under enough coats of paint so as to resist total removal, install a sheet of 3/8-inch-thick dry wall on the ceiling, nailing it securely to the ceiling joists. ''Set'' the nailheads. Then fill the nail holes with regular dry-wall ''mud,'' sand smooth, and apply a coat of oil-based primer sealer.
Finally, apply the wallpaper following the manufacturer's directions precisely.