Prep sanding drywall, and plaster walls and ceilings improves adhesion of new paint coats to existing hard, glossy surfaces such as those painted with oil and water base enamels. These non-porous finishes are typically found in kitchens, bathrooms, and laundry rooms. To improve adhesion for heavy textured walls and ceilings, etching with liquid sandpaper or applying a coat of primer may be good alternatives.
Prep sanding is also the only practical way to smooth over unsightly blemishes such as paint runs, uneven fillers, heavy brush marks, and thick roller nap. Sanding is also effective at removing sharp points and ridges that sometimes accompany wall textures.
Sanding drywall and plaster is most easily accomplished with the use of a pole sander. This tool is designed to provide the user with maximum leverage to sand large areas. Pole sanders use pre-designed drywall sandpaper or standard sandpaper sheets cut to the proper size. A quick way to prepare a standard 9x11 inch sheet of sandpaper is to fold it in half and cut it into two 4-1/2x11 inch sheets. An individual sanding head with an extension pole can be a substitute for a pole sander.
To begin sanding, place the sanding head flat against the wall and firmly grasp the extension pole with two hands. Work the sanding pole in an up and down motion. Some situations may exist where a side to side motion is preferable. Be careful not to let the sanding head pivot onto its side. That can easily happen when the sandpaper suddenly grips the paint.
Always finish off with medium grit sandpaper, such as 120-150 grit. Coarser sandpaper may leave sanding scratches that can show through the new coats of paint.
If the areas to be sanded are limited to a few filler spots, a sanding sponge or a folded piece of sandpaper is a convenient choice. Sanding sponges are available in coarse, medium, and fine grits.
Keep in mind that sanding dust has the tendency to drift around and settle on everything. By following some simple masking procedures, you can avoid unnecessary clean-up.
Medium sandpaper, 120-150 grit is a good choice for sanding previously painted walls and ceilings in good condition. Some examples include the following:
It is generally a good idea to sand any wall or ceiling before applying a new coat of paint. A light sanding is usually sufficient for medium and heavy textured walls and ceilings.
Coarse sandpaper, 80-100 grit can be used for sanding drywall, and plaster walls and ceilings in less than good condition. Some examples include the following:
Loose, peeling paint should be scraped away before sanding. Once the loose paint has been removed, the paint edges should be feather sanded to remove uneven ridges. If the underlying substrate is drywall, care should be taken not to over sand it. Coarse sandpaper can perforate the paper on the surface of exposed drywall. For that reason, medium grit sandpaper may be a better choice for feather sanding paint ridges.