Patching Hairline Cracks In Plaster

Procedure code:
Hspg Prepared For Nps - Sero
Gypsum Plaster
Last Modified:



  1. This procedure includes guidance on patching hairline cracks in plaster with reinforcing tape and joint compound.
  2. Cracks may be cyclical, opening and closing with seasonal variation in humidity which causes the lath to swell and shrink.
  3. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
    1. Safety Precautions
    2. Historic Structures Precautions
    3. Submittals
    4. Quality Assurance
    5. Delivery, Storage and Handling
    6. Project/Site Conditions
    7. Sequencing and Scheduling
    8. General Protection (Surface and Surrounding)

These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).


  1. Environmental Requirements:
    1. Keep the room temperature above 55 degrees F until the plaster/joint compound has set.
    2. Provide plenty of ventilation as the plaster dries.



  1. USG Corporation
  2. TKO Waterproof Coatings, LLP


  1. Joint compound such as "Sheetrock Setting-Type Joint Compound" (USG Corp.), "Krack-kote" (TKO Waterproof), or approved equal.
    1. "Krack-kote": Good for problem cracks that may break through the Sheetrock tape and compound.
      1. It uses a pliable adhesive and a glass fiber reinforcing tape; it has more flexibility and strength than ordinary joint compound.
      2. Available from large paint supply stores.
      3. It is more expensive and more timely to apply than ordinary joint compounds.
  2. Reinforcing tape (cloth or paper): Cloth is better for flat surfaces because of its open-weave, but it is difficult to find in the U.S.
  3. Acrylic latex caulk


  1. Wide joint knife (approximately 5-6 inches wide)
  2. Sponge or heavy-nap cloth
  3. Caulking gun
  4. Crack widener or triangular can opener
  5. Stiff bristle brushes or vacuum



  1. Types of plaster cracking include, map cracking, alligatoring, settlement cracks, hairline cracks, stress related cracks and cracks due to moisture.
  2. If a wall has an enormous number of cracks to be taped, consider replastering or canvasing the surface.


  1. Slightly widen the crack with a sharp, pointed tool like a crack widener or a triangular can opener.
  2. Brush or vacuum surface to remove dust and debris.
  3. Apply joint compound with a wide joint knife; Butter the compound into the crack, spreading it about 3 inches on either side of the crack.
  4. Center mesh reinforcing tape over the crack, and force the tape down into the bed of the joint compound with the knife; Remove any excess compound by wiping with the joint knife.
  5. When the tape is bedded, cover surface with a thin layer of compound and smooth as much as possible by working with the joint knife.
  6. When the first coat has dried (at least 24 hours), smooth out any ridges by "wet sanding" with a damp sponge or a heavy-nap cloth folded flat or wrapped around a suitable block.
  7. Apply a second thin coat of joint compound and feather the edge at least 1 inch beyond the first coat.
  8. After the second coat has dried, wet-sand lightly and apply a thin finishing coat.
  9. Lightly sand the surface again, and clean off the area with damp sponge.
  10. After the surface has dried, brush off any plaster residue or dust.

NOTE: For gaps between plaster surfaces and surrounding woodwork, apply acrylic latex caulk using a caulking gun.

Last Reviewed: 2017-12-11