Check the drywall for straightness by placing a level against the wall. Replace warped drywall if there is a lot of uneven surface causing the wall to look blotchy. Cut drywall with a jigsaw.
Screw in new drywall with 1/2-inch drywall screws and a No. 2 Phillips screwdriver. Apply joint compound and tape to the seams. Also, spread joint compound over the heads of the screws. Seal the wall with primer and paint.
Apply primer to the wall if the blotchy discoloration comes from the previous color showing through the new paint. Use gray primer to cover over dark paint or white primer to cover over light-colored paint. Allow it to dry.
Apply two coats of paint over the wall. Place the roller at the top of the wall. Move downward in a continuous motion to prevent lines where the paint overlaps.