Fixing Hairline Cracks
Moisten the crack with water using a spray bottle or garden hose. Start this process a few hours before you plan to start your wall repair, and add more water once an hour until you are ready to begin. This moisture will help the patch dry slowly and reduce the risk of further cracks.
Make a grout mixture by blending Portland cement and water. Follow the directions on the Portland cement package you've chosen, taking care not to add too much water. Properly blended grout should form a thick paste and should not be overly wet or liquidy.
Use a pointing trowel or putty knife to force grout into the crack. Be sure to fill the entire crack, then wipe away excess grout so that it is level with the rest of the wall surface.
Tape a sheet of plastic over the patched area. Lift the plastic once a day to spray the patch with water, then re-cover. Repeat this process for five days, then remove the plastic and paint the wall as desired.
Repairing Large Cracks or Holes
Undercut the crack using a hammer and chisel. Chip away enough concrete so that the bottom of the crack is wider than the surface. This inverted V-shape will help hold the patch in place as it dries.
Sweep the crack clean with a wire brush, making sure to remove all loose stones and dust. Work the brush into the corners of the crack to make sure it is completely clean before proceeding.
Paint the inside of the crack with a cement bonding adhesive. This material will help the patching compound bond with the existing cement. It also prevents the existing cement from soaking in too much water from the new patch, which can minimize the risk of future cracks in the area.
Blend ready-mix concrete with water in a bucket or wheelbarrow. Follow the instructions on the package when determining how much water to add, and use a drill with a paddle bit to blend the mixture thoroughly.
Use a pointing trowel to fill the crack, making sure to fill it completely. You may wish to work in layers to ensure the entire void is filled. Wipe away any excess mixture so that the patched area is smooth and level with the rest of the wall. Use your trowel or a broom to create texture on the patch so that it matches the rest of the wall.