Before applying wallpaper or a new coat of paint, it’s important to repair any cracks in the plaster wall beneath. Start by visiting a hardware or home repair store for patching plaster, joint compound and a putty knife or spackling tool. If the wall contains very small hairline cracks, carefully dig them out to make them a little wider. This gives the patch something to adhere to. Start with a clean, dry surface. After mixing the patching plaster according to the manufacturer’s directions, use your putty knife or spackling tool to work the mixture into each crack or hole in the wall. It’s important to put as much of the mixture as possible inside the crack to help the patch stick. When the crack or hole is full, carefully smooth out the surface and scrape away any stray bits of plaster that might have fallen onto the wall. Let the patch dry for 24 hours, then check to see if it has receded further into the wall. If so, simply add more patching plaster to make the wall surface even again, and let it dry for another 24 hours. If necessary, use a good grade of sandpaper to smooth out the surface before applying paint or wallpaper.