Things You'll Need
4-inch, 6-inch and 10-inch putty knives
Fine-grit sanding sponge
Hairline cracks can often be found in homes with plaster walls because plaster can shrink with age. Also, the wood lath and studs easily absorb humidity in the environment, which causes the wood to swell and shrink. This can cause slight movements in the plaster and result in hairline cracks. Patching these cracks is a fairly easy process and if the repair is done correctly, the crack won't ever come back.
Look at the crack and pull out any tiny bits of plaster inside the crack.
Run the utility knife along both sides of the crack to cut out any chipped paint or bits of plaster that are sitting higher than the rest of the wall surface.
Measure the length of the crack, then measure out a piece of mesh tape that is the correct length. Apply the mesh tape to the wall and run your hand over the tape so there are no air bubbles or ridges in the tape. It's important that the middle of the mesh tape is directly over the crack so there is the same amount of tape on both the left and right side of the crack.
Open the container of joint compound and fill the mud pan.
Load the tip of the 4-inch putty knife with joint compound and run the knife over the taped crack. Multiple thin layers of compound will give a much better result than one thick layer, so try to keep the first layer as close to 1/8 inch as you can. Allow this layer to dry.
Load the 6-inch putty knife with joint compound and run it over the first coat of patch. This time, you should make three passes over the cracked area. The putty knife should be left of center for the first pass and right of center on the second pass. The knife should be directly over the center of the patched area for the third pass. Allow this coat to dry.
Run the 10-inch putty knife over area you patched to remove any larger chunks of dried joint compound.
Scoop joint compound out of the mud pan with the 10-inch putty knife, then make three passes over the cracked area, using the same technique you used for the second coat.
Sand the area once the third layer is dry. Use very little pressure with the sanding sponge to avoid sanding off too much of the patched area. Light pressure is all that's needed to knock down any high or low spots on the wall.
Lightweight joint compound works best when patching over plaster walls. While it takes longer to dry than quick-setting compound, it is much easier to sand. Once the hairline crack is repaired, you'll need to prime and paint the wall to complete the job.