Three-Coat Plaster Patching Holes
- Procedure code:
- Hspg Prepared For Nps - Sero
- Gypsum Plaster
- Last Modified:
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
A. This procedure includes guidance on patching holes in plaster. The extent of work described includes the following:
- Removing deteriorated plaster, and
- 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:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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 www.usg.com B. TKO Waterproof Coatings, LLP www.tkocoatings.com
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.
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:
- 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.
- Chipping, flaking and delamination of plaster due to water infiltration.
3.02 ERECTION, INSTALLATION, APPLICATION
A. Removing Deteriorated Plaster:
- 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").
- Wear a dust mask, goggles and gloves and pull loose plaster from the walls with your hands or a flat prybar.
- 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.
- Cut the plaster back to the studs, if necessary, and re-secure the lath with drywall nails.
- Knock any plaster stuck between the lath back into the wall cavity.
- 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:
- 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).
- 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.
- Use tie wire to secure the metal lath over the wood lath.
- 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.
- To secure the lath at studs, drive 1 inch drywall screws between the lath into the stud.
C. Applying the Scratch and Brown Coats:
- Thoroughly moisten the old wood lath with a spray bottle, so it will not draw moisture out of the wet plaster used for patching.
- 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).
- 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.
- Apply the second coat (brown coat) using the same plaster that was used for the scratch coat.
- 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.
- 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:
- Place approximately 2 quarts of cold, potable water into a 5 gallon plastic bucket.
- Scoop in autoclave finish lime until it starts to float. NOTE: WEAR GOGGLES, A DUST MASK OR RESPIRATOR, AND LATEX GLOVES WHEN MIXING LIME.
- If using single-hydrated lime, sift it into the water the day before and leave to slake overnight.
- Mix thoroughly with a mixer attachment on an electric drill, or by hand with a pointing trowel.
- Add small amounts of water or lime as necessary until all the lime is wet and there are no lumps or standing water.
- Use a scrap of plywood for a mortarboard and toss some lime putty onto it.
- Form the putty into a ring using the margin trowel; Fill the center of the ring about 2/3 full with cold, clear water.
- 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.
- Mix the water and the plaster in the middle of the ring; It should be slightly stiffer than the lime putty.
- Fold in the lime putty and mix until all of the ingredients are stirred up.
E. Applying the Finish Coat:
- 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.
- Following the same motions used when applying the first two coats, trowel the finish coat onto the patch.
- Fill in any hollows by dabbing with plaster and smooth out ridges.
- 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.
- Let the finish coat cure for about 1 week; Check it for shrinkage.
- 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.
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.