A properly-finished drywall repair creates its own disguise by blending into the rest of the wall or ceiling, but that’s an ideal scenario. If you have to make a repair on a surface with an existing texture, for example, it can be next-to-impossible to copy the texture, because application techniques vary so widely. Your best bet is often to skim coat the surface and start over. If there is no texture, hiding the repair is simply a matter of proper seam finishing, which sometimes requires several coats of mud applied with increasingly wide drywall knives.
Hide a repair on an untextured wall by coating it with drywall joint compound, or mud, and scraping the mud flat with a drywall knife. After it dries, recoat it and scrape that coat with a longer knife. Repeat a third and fourth time if necessary. The knives should range in width from 4 to 12 inches.
Finish seams on a textured wall in the same way, coating with several coats of mud and scraping each coat with a longer blade. When the final coat dries and seam is flat, apply a texture coat identical to the existing one. This is more difficult than it sounds, because there is a wide variety of texturing techniques.
Roll diluted joint compound with a thick-nap paint roller to copy an eggshell or other close-patterned texture.
Spray diluted joint compound with a hand sprayer to copy popcorn or knockdown patterns with widely-separated mottles. You can also use spray texture from a can to copy some patterns. To knock the pattern down, or flatten it, wait for the mud to stiffen, the scrape over it lightly with a drywall knife.
Skim coat the entire surface to erase the texture pattern if you can’t copy it accurately. To skim coat, spread undiluted joint compound over the entire wall and scrape it flat with an 8-inch drywall knife. Let the mud dry, then apply a second coat. Sand the final coat with 120-grit sandpaper after it dries.
Apply a new texture over the skim-coated surface, using whatever technique you wish, but keep an eye on the texture on the walls in the rest of the room and try to match it. If you can’t make an exact match, that fact probably won’t be noticeable, as long as there are no variations on the wall you repaired.
Prime the repair with drywall primer and then paint the entire wall on which you made the repair. If you can still see vestiges of the repair, use semi-gloss paint. It scatters and deflects light, making inconsistencies in the surface less noticeable.