How to Repair Wall Cracks
To repair cracks in your drywall, buy prepared joint compound or mix some yourself using the instructions provided on setting-type compound powder, which you can get from the hardware store. Next, cut a V-notch 1/4 to 1/8 inch along the crack to help keep the compound in place and remove any debris inside the crack with a vacuum hose. Using a putty knife, fill the crack by applying as many coats of joint compound as you need. For the best results, let the compound dry completely between coats. Once the final layer has dried, sand the area with medium-grit sandpaper until it's smooth, then paint over the crack with latex primer and wall paint.
Buy prepared joint compound or mix it yourself. If you buy it premade, look for all-purpose compound which can be used for all coats. If you make your own, use setting-type compound powder and follow the instructions provided on the container. All of these compounds can be found at a hardware store. Setting-type compound powder is more difficult to sand and to prepare properly so it is not the best option for beginners. Cut a V-notch 1⁄4 to 1⁄8 inch (0.64 to 0.32 cm) along the crack. The “V” shape will help keep the compound in place. Remove any debris inside the crack with a vacuum hose or by simply scraping it out. Apply coats of joint compound over the crack. Use a 3 to 4 in (7.6 to 10.2 cm) putty knife. Let the compound dry completely between coats. Layer on as many coats as necessary to fill the crack. The average is 3 coats. Depending on what type of compound you use, it can take anywhere from 45 minutes to 24 hours to dry. If the cut is deeper than ⁄ 4 inch (0.64 cm), you may need to press a strip of mesh or paper tape into the first layer of compound before it dries to better seal the crack. Thinner coats are ideal as they're easier to sand down to match the wall once dry. Sand the dried joint compound with medium-grit sandpaper. Use a sanding block to smooth out the section to the flatness of the wall. Always wear a dust mask when sanding to avoid inhaling particles. 80-grit (medium-grit) sandpaper can eliminate larger bumps whereas a finer 120-grit can be used for finishing touches. One option is to sand in between coats to avoid having to sand as much at the end. Paint over the crack with latex primer followed by wall paint. If you don't use primer first, your patched area won't blend in properly with the rest of the wall. The exception to this is if you used a paint and primer in one. Then you only need a coat or two of the paint directly on the affected area. Press gently on the wall near the crack to see if it gives. If it's easy to press down or if the wall flexes at all, the plaster has likely come apart from the lath. This requires reattaching the two together and cannot be fixed by a simple patch job. Contact a professional for help. Widen the crack using a putty knife if it's less than 1⁄4 inch (6.4 mm) wide. It sounds counterproductive but scoring (lightly scratching) the crack creates a wider surface for the joint compound to more easily adhere to. Spread a coat of joint compound over the crack. Do so with a 4 in (10 cm) taping knife. Use a setting-type compound instead of drywall compound as it is stronger and can affix to plaster more securely. Apply in thin layers. Dampening the crack before applying the compound will remove any loose particles and help the compound hold better. Press fiberglass mesh tape firmly into the wet compound over the crack. Cut the strips to size with scissors. This self-adhesive tape will prevent the crack from growing. Let dry. For the setting-type compound to dry thoroughly, your room should be between 55 and 70 °F (13 and 21 °C). Apply 3 layers of compound over the taped area. Let each coat dry completely in between. With each additional layer, extend the compound another 2 to 3 inches (5.1 to 7.6 cm) outside the previous layer's edges. Your last layer should extend 12 inches (30 cm) beyond the original area. Lightly sand each layer with a fine sandpaper to remove bumps in between coats. Use a feathering technique when applying the compound. With the knife at a 70-degree angle, start at the center and pull the knife to the outside edges of each coat, increasing pressure the further away from the middle you get. Paint over the patched area to match the rest of the wall. If you can see a raised section where you did your repair, sand it flush to the wall before painting so it blends seamlessly. It's smart to wait at least 24 hours prior to painting to ensure the compound is completely dry. Enlarge the crack with a chisel and hammer. A technique known as undercutting (which is essentially chipping away at the concrete) should be done to 1 inch (2.5 cm) below the edges of the crack. This provides more surface area for the patching material to grip. Clean debris from the crack using a wire brush. A vacuum can also help get rid of dust, pieces of concrete, or dirt lurking in the crack. Rinse it with water and dry thoroughly with a towel. If you don't have a wire brush, a flat head screwdriver or air compressor will work. Prime the area with a concrete bonding adhesive. This will help the patching material adhere better to the concrete. You'll want to use an old paintbrush to spread a thin layer around the edges and deep into the crack. Apply multiple coats of concrete patching with a putty knife. Press each layer into the crack and let dry completely in between coats. Repeat until the crack is filled and level with the rest of the wall. You can also use a pointed trowel if you don't have a putty knife. This is good for pushing the patching into the crack. Add texture to the patched area before it dries. Matching new concrete to old concrete is difficult because it's so smooth. Rough the last layer up with a broom or any tool while it's still wet to make it look more worn. Sealing the patch with a heavy-duty water-based polyurethane with a brush can prevent stains and other marks. 
Buy prepared joint compound or mix it yourself. Cut a V-notch 1⁄4 to 1⁄8 inch (0.64 to 0.32 cm) along the crack. Apply coats of joint compound over the crack. Sand the dried joint compound with medium-grit sandpaper. Paint over the crack with latex primer followed by wall paint. Press gently on the wall near the crack to see if it gives. Widen the crack using a putty knife if it's less than 1⁄4 inch (6.4 mm) wide. Spread a coat of joint compound over the crack. Press fiberglass mesh tape firmly into the wet compound over the crack. Apply 3 layers of compound over the taped area. Paint over the patched area to match the rest of the wall. Enlarge the crack with a chisel and hammer. Clean debris from the crack using a wire brush. Prime the area with a concrete bonding adhesive. Apply multiple coats of concrete patching with a putty knife. Add texture to the patched area before it dries. 