Historic Preservation - Technical Procedures
- Three-Coat Plaster Patching Holes
- Procedure code:
- Hspg Prepared For Nps - Sero
- Gypsum Plaster
- Last Modified:
- Three-Coat Plaster Patching Holes
- Last Modified:
THREE-COAT PLASTER PATCHING HOLES
A. This procedure includes guidance on patching holes in
plaster. The extent of work described includes the
1. Removing deteriorated plaster, and
2. Applying three coats of plaster to the damaged
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
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
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).
A. USG Corporation
B. TKO Waterproof Coatings, LLP
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
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
S. Paint brush
A. Determine the extent of unsound plaster and evaluate work
requirements and causes before proceeding. Signs of
damaged plaster in need of replacing include:
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
2. Wear a dust mask, goggles and gloves and pull loose
plaster from the walls with your hands or a flat
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
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
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
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
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
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
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
6. Tape any small cracks with cloth mesh tape and
F. Wash down new plaster with a zinc sulphate solution (2
lbs./gallon of water) and allow to dry.
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
END OF SECTION