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Historic Preservation - Technical Procedures

Spectitle:

Patching Hairline Cracks In Plaster

Procedure code:

0921002R

Source:

Hspg Prepared For Nps - Sero

Division:

Finishes

Section:

Gypsum Plaster

Last Modified:

02/24/2012

Details:

Patching Hairline Cracks In Plaster



PATCHING HAIRLINE CRACKS IN PLASTER<= br>

PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on patching hairl= ine
         cracks in plaster with reinforcing tape and joint
         compound.

    B.   Cracks may be cyclical, opening and closing with seasonal
         variation in humidity which causes the lath to swell and
         shrink.

    C.   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 Precaution= s

         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).

1.02 PROJECT/SITE CONDITIONS

    A.   Environmental Requirements:

         1.   Keep the room temperature above 55=F8F until the
              plaster/joint compound has set.

         2.   Provide plenty of ventilation as the plaster dries.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   U.S. Gypsum Association
         810 First Street NE, #510
         Washington, DC  20002
         202/289-5440, FAX 202/289-3707

    B.   Tuff-kote Company, Inc.
         210 Seminary Avenue
         Woodstock IL  60098
         815/338-2006

2.02 MATERIALS

    A.   Joint compound such as "Durabond Wallboard Compound"
         (U.S. Gypsum Association), "Krack-k= ote" (Tuff-Kote Co.),
         or approved equal.

         1.   "Krack-kote":  = Good for problem cracks that may
              break through the Sheetro= ck tape and compound.

              a.   It uses a pliab= le adhesive and a glass fiber
                   reinf= orcing tape; it has more flexibility and
                   stren= gth than ordinary joint compound.

              b.   Available from large paint supply stores.

              c.   It is more expensive and more timely to apply
                   than ordinary joint compounds.

    B.   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.

    C.   Acrylic latex caulk

2.03 EQUIPMENT

    A.   Wide joint knife (approximately 5-6 inches wide)
    B.   Sponge or heavy-nap cloth

    C.   Caulking gun

    D.   Crack widener or triangular can opener

    E.   Stiff bristle brushes or vacuum


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Types of plaster cracking include, map cracking,          alligatoring, settlement cracks, hairline cracks, stress
         related cracks and cracks due to moistur= e.

    B.   If a wall has an enormous number of cracks to be taped,
         consider replastering or canvasing the surface.

3.02 ERECTION, INSTALLATION, APPLICATION

    A.   Slightly widen the crack with a sharp, pointed tool like
         a crack widener or a triangular can opener.

    B.   Brush or vacuum surface to remove dust and debris.=

    C.   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.

    D.   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.

    E.   When the tape is bedded, cover surface with a thin layer
         of compound and smooth as much as possib= le by working
         with the joint knife.

    F.   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.

    G.   Apply a second thin coat of joint compound and feather
         the edge at least 1 inch beyond the first coat.

    H.   After the second coat has dried, wet-sand lightly and
         apply a thin finishing coat.

    I.   Lightly sand the surface again, and clean off the area
         with damp sponge.

    J.   After the surface has dried, brush off any plaster=
         residue or dust.

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

                             END OF SECTION