Repair the Crack

• 1). Cut away the edges of the cracked plaster with a utility knife. Keep cutting along each edge until you reach plaster that is still secure to the substrate beneath and is not chipped or peeling.
  
  
• 2). Mix the joint compound, following the directions on the packaging for your particular product. The proportions of water to joint compound mix may vary from product to product. Prepare the compound in a plastic bucket and work the mixture with the garden trowel until it is smooth and uniform.
  
  
• 3). Spread the joint compound over the crack using a 4-inch drywall taping knife. Keep the application of joint compound even and approximately 1/8-inch thick. Cut a piece of drywall tape to the length of the crack and lay it over the drywall compound, smoothing the surface with your gloved fingers.
  
  
• 4). Drag the side edge of the 4-inch taping knife along the length of the drywall tape. The pressure will remove the excess joint compound, approximately 50% of what you applied, and the edge of the knife will remove it. Allow the joint compound to cure for the amount of time recommended by your product.
  
  
• 5). Spread a second layer of joint compound over the first layer and the tape. Use the 6-inch drywall taping knife and make this layer only half as thick as the first. Repeat the taping and smoothing process that you used previously.
  
  
• 6). Let the joint compound set up again. Lightly mist the surface with a spray bottle and drag the flat face of a rubber grout float along the patched crack to smooth the area. Let the area dry, then apply a third, very thin layer of joint compound with the 10-inch drywall knife.
  
  

Skim Coat Over the Plaster

• 1). Let the repair dry completely over 24 to 48 hours. Prepare the room as you would if you were painting, covering the floors with drop cloths and taping off trim and doorways with painter’s tape. Remove portable items from the area and cover what cannot be moved with plastic and masking tape.
  
  
• 2). Mix up the drywall compound with the recommended amount of water in a clean plastic bucket. Work the drywall compound with a clean garden trowel until it is uniform throughout and there are no lumps. Use your plaster trowel to scoop at least several cups of prepared compound onto your plasterer’s hawk.
  
  
• 3). Apply the drywall compound from the top of the wall toward the center using your trowel. Imagine that there is a horizontal line splitting the wall in half, and stop there. Make this layer about 1/16-inch thick and even all over and add more compound to your hawk as necessary.
  
  
• 4). Spread compound from the bottom of the wall up to the center so that the applications meet in the middle. Smooth the center line with your trowel. Let this layer of compound cure according to the package directions.
  
  
• 5). Apply a second layer of drywall compound the same way you did the first, but make this application about 1/8-inch thick. Smooth any rough or uneven areas of the wall with the flat face of your trowel and let the compound dry 24 to 48 hours (or longer, if recommended by your product.) You can now sand and paint your walls as desired.