Thoroughly clean the walls before painting or wallpapering. This is an often overlooked step, but doing so will ensure better finishing results.
If the wall had old wallpaper, a residue of old glue probably remains which should be removed. If it's just old paint, there's probably a layer of grime that prevents new paint from bonding.
In either case, the best cleaning agent is often TSP (trisodium phosphate). Mix TSP with water and sponge it over the walls and woodwork.
NOTE: TSP is somewhat caustic, and will actually remove the gloss from old paint. That helps new paint because glossy surfaces don't bond as well. But be careful not to get TSP on painted surfaces you want to save. As always, follow all package directions.
Ripping off old wallpaper may reveal wall damage that the paper was hiding. Such areas should be repaired before moving on to the finishing stages.
Narrow cracks and small holes are easily repaired with joint compound, or ready-mix spackling compounds sold in small containers at home centers.
Use a putty knife to apply the material, daubing on enough to fill the damaged area. Wipe away any excess with a rag or damp sponge and allow to dry. Some products may shrink as they dry and require a second application to fill completely. On wider cracks, remove any loose paint, plaster or drywall material before patching.
You can also reinforce the repair by laying a strip of fiberglass mesh tape over the crack and embedding that in the patching material. After it dries, apply a second coat of compound to cover the tape.
Repairing Larger Holes
On larger holes, you can use metal mesh repair patches with a sticky backing that adheres to the wall surrounding the hole. Finish that patch by covering it with spackling or drywall joint compound, applied in two or three thin layers.
Another option is to patch it with a piece of drywall. First, trim the drywall back from the damaged area, leaving a rectangular area to fill. Then you need some kind of backer inside the wall to support the patch. You can set one or two plywood strips in behind the hole and secure them by screwing into them through the front of the drywall.
- Cut the patch to fit the damaged area.
- Screw that into the backer.
- Fill the joints between the patch and the existing wall with joint compound.
- Embed mesh tape over the joints to reinforce them.
- Finish the joints with one or two more layers of compound after the first layer dries.
After removing wallpaper and/or patching cracks and holes, lightly sand the entire area to be painted.
If patched areas dried especially rough and don't respond to light sanding, try using a 6-inch taping knife as a scraper to knock off the higher, rougher spots of dried joint compound. Use caution though, because it's easy to gouge the patched area with the corners of the blade.
Then use the open weave type of sand paper designed for drywall surfaces to finish smoothing out the patches.