Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Three-Coat Plaster Patching Holes

Procedure code:

0921003R

Source:

Hspg Prepared For Nps - Sero

Division:

Finishes

Section:

Gypsum Plaster

Last Modified:

03/14/2014

Details:

Three-Coat Plaster Patching Holes



THREE-COAT PLASTER PATCHING HOLES


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on patching holes in
         plaster.  The extent of work described includes the
         following:

         1.   Removing deteriorated plaster, and

         2.   Applying three coats of plaster to the damaged
              area.

    B.   Most buildings built before 1914 had hair plaster
         (plaster with animal hair mixed in to provide greater
         strength); Plaster installed after 1914 may have a fiber
         bonding agent rather than animal hair...or, it may have
         no bonding agent at all.

    C.   Complete failure of plaster, requiring a three-coat
         plaster patch, is usually localized around doors and
         windows, on stair soffits or areas severely damaged by
         water infiltration.

    D.   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).


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   USG Corporation
         www.usg.com
         
    B.   TKO Waterproof Coatings, LLP
         www.tkocoatings.com
         

2.02 MATERIALS

    A.   Metal Lath

    B.   18 Gauge Tie Wire and Drywall Nails

    C.   Scratch and brown coats:  Perlite gypsum plaster, such as
         "Structo-Lite" (USG.com), or approved equal.

    D.   Finish Coat Gauging Plaster, Finish Lime

    E.   Drywall screws

    F.   Cloth mesh tape

    G.   Joint compound such as "Krack-Kote" (tkocoatings.com), or
         approved equal.

2.03 EQUIPMENT

    A.   Hawk

    B.   Slicker (flexible straight-edge)

    C.   Plasterer's trowel

    D.   Margin trowel

    E.   Mortarboard and mudpan

    F.   Pointing trowel

    G.   Plywood scrap

    H.   6 inch taping knife

    I.   12 inch taping knife

    J.   Joint tape (cloth mesh preferred)

    K.   Stiff putty knife

    L.   Goggles, work gloves, and dust mask

    M.   Hammer and cold chisel

    N.   Needlenose pliers and wire cutter

    O.   Screwgun and drill

    P.   Spray bottle and dropcloths

    Q.   Tin snips

    R.   Shop-vac

    S.   Paint brush


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Determine the extent of unsound plaster and evaluate work
         requirements and causes before proceeding.  Signs of
         damaged plaster in need of replacing include:

         1.   Holes

         2.   Water Stains:  Brownish rings on the plaster,
              especially the ceilings, indicate that the plaster
              has been wet.  If the water was stopped quickly,
              the surface may only need to be sealed with
              pigmented shellac to prevent the stain from
              bleeding through the new paint or wallpaper.
              However, if the leak continued for a long period,
              the plaster may need to be replaced, and will often
              have a powdery appearance.

         3.   Chipping, flaking and delamination of plaster due
              to water infiltration.

3.02 ERECTION, INSTALLATION, APPLICATION

    A.   Removing Deteriorated Plaster:

         1.   Use plaster washers to re-secure weakly-keyed areas
              of otherwise sound plaster to the wall or ceiling
              (see 09210-04-R "Resecuring Loose Wall or Ceiling
              Plaster").

         2.   Wear a dust mask, goggles and gloves and pull loose
              plaster from the walls with your hands or a flat
              prybar.

         3.   To remove sound plaster, drill holes in the line of
              cut with a carbide drill bit; Holding the chisel at
              a shallow angle, carefully cut directly from hole
              to hole with a cold chisel; Cut the resulting
              plaster free from the lath by chipping the keys
              from the side.

         4.   Cut the plaster back to the studs, if necessary,
              and re-secure the lath with drywall nails.

         5.   Knock any plaster stuck between the lath back into
              the wall cavity.

         6.   Vacuum all dust, loose plaster, and other debris
              from the hole with a shop-vac, or brush it out with
              an old paintbrush.

    B.   Lathing the Hole:

         1.   Install metal lath over the wood lath (metal lath
              lessens the likelihood of cracking caused by the
              old wood lath drawing too much moisture out of the
              plaster).

         2.   If 2 hands are necessary to secure new metal lath,
              drive a finishing nail into an exposed stud; take a
              piece of lath slightly larger than the hole and
              hang it on the nail; with both hands now free, cut
              the lath to shape with tin snips.

         3.   Use tie wire to secure the metal lath over the wood
              lath.

         4.   To make a tie wire, bend a 6 inch long piece of
              wire into an elongated "U" and pull it around the
              old wood lath; space the tie wires every 6 inches.

         5.   To secure the lath at studs, drive 1 inch drywall
              screws between the lath into the stud.

    C.   Applying the Scratch and Brown Coats:

         1.   Thoroughly moisten the old wood lath with a spray
              bottle, so it will not draw moisture out of the wet
              plaster used for patching.

         2.   Apply the base coat (scratch coat) of plaster to
              the wall in an arching motion; Keep the hawk close
              to the wall under the trowel to catch falling
              plaster; Use a margin trowel to work the plaster
              into edges and corners.  The thickness of the new
              scratch coat should not exceed that of the old
              scratch coat (about 1/8 to 1/4 inch).

         3.   As it starts to set, score shallow, random
              scratches in it diagonally about every inch or so
              to provide keys for the next layer; Let the scratch
              coat set for 48 hours.

         4.   Apply the second coat (brown coat) using the same
              plaster that was used for the scratch coat.

         5.   Run a slicker (a long flexible straight edge) over
              the entire patch; Keep the brown coat below the
              level of the surrounding finish coat by about 1/8
              inch.

         6.   As the brown coat starts to set, knock off any high
              spots; Let the brown coat set for 48 hours before
              applying the finish coat.

    D.   Mixing the Finish Coat:

         1.   Place approximately 2 quarts of cold, potable water
              into a 5 gallon plastic bucket.

         2.   Scoop in autoclave finish lime until it starts to
              float.  NOTE:  WEAR GOGGLES, A DUST MASK OR
              RESPIRATOR, AND LATEX GLOVES WHEN MIXING LIME.

         3.   If using single-hydrated lime, sift it into the
              water the day before and leave to slake overnight.


         4.   Mix thoroughly with a mixer attachment on an
              electric drill, or by hand with a pointing trowel.

         5.   Add small amounts of water or lime as necessary
              until all the lime is wet and there are no lumps or
              standing water.

         6.   Use a scrap of plywood for a mortarboard and toss
              some lime putty onto it.

         7.   Form the putty into a ring using the margin trowel;
              Fill the center of the ring about 2/3 full with
              cold, clear water.

         8.   Slowly sprinkle in gauging plaster until all the
              water is absorbed; The mix should consist of 1 part
              gauging plaster to 3 parts lime putty.

         9.   Mix the water and the plaster in the middle of the
              ring; It should be slightly stiffer than the lime
              putty.

         10.  Fold in the lime putty and mix until all of the
              ingredients are stirred up.

    E.   Applying the Finish Coat:

         1.   If applying the finish coat over an old brown coat,
              the existing brown coat must be moistened well
              before applying the finish coat (unnecessary on a
              newly-applied brown coat).  

         NOTE:  Finishing plaster over an old brown coat
         occasionally fails because the new plaster does not bond
         to the dry, old substrate, even if it is wet down.
         Furthermore, the old plaster will absorb water faster,
         causing cracks in the finish coat as it starts to dry.

         2.   Following the same motions used when applying the
              first two coats, trowel the finish coat onto the
              patch.

         3.   Fill in any hollows by dabbing with plaster and
              smooth out ridges.

         4.   Spray a fine mist of water onto the plaster and
              trowel over the patch again to get a slick finish;
              Straighten edges and corners with margin trowel.

         5.   Let the finish coat cure for about 1 week; Check it
              for shrinkage.

         6.   Tape any small cracks with cloth mesh tape and
              joint compound.

    F.   Wash down new plaster with a zinc sulphate solution (2
         lbs./gallon of water) and allow to dry.

3.03 ADJUSTING/CLEANING

    A.   Remove all rubbish and debris caused by plastering work.

    B.   Clean all affected surfaces of room and furnishings to
         their prior condition.

    C.   Remove temporary protection and enclosure of other work.
         Promptly remove plaster from door frames, windows, and
         other surfaces that are not to be plastered.  Repair
         floors, walls, and other surfaces that have been stained,
         marred, or otherwise damaged during the plastering work.
         When plastering work is completed, remove unused
         materials, containers, and equipment and clean floors of
         plaster debris.

                             END OF SECTION
 


traditional plaster, three-coat plaster, three coat plaster, traditional three coat plaster, patching plaster holes