Patching Weathered, Exfoliated, Or Blistering Sandstone

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


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.



  1. This procedure includes guidance on composite patching of sandstone. Composite patching is required when portions of the stone surface are lost and must be replaced. For retaining sandstone that is delaminating internally, see 04470-02-R "Repairing Sandstone by Through Surface Repair".
  2. Composite patching is the process whereby cement and sand mixtures are applied as a series of stucco-like coats to reconstruct missing stone surfaces. Three types of stone deterioration that warrant composite patching include weathering, exfoliation, and blistering.
  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).
  4. For general information on the characteristics, uses and problems associated with sandstone, see 04470-01-S.


  1. American Society for Testing and Materials (ASTM)


  1. Samples: Routine testing of materials, of proposed mortar mix, and of final work for compliance with this procedure will be carried out by the RHPO or his\her appointed representative.
    1. Select sand and aggregate resembling a crushed sample of the stone to be matched; Blend different sands and pigments with crushed stone as necessary.
    2. Keep track of the amounts added to each sample; The color of the dry mix is a good indication of the final repair color.
    3. Add water and an acrylic latex admixture to make a paste sample large enough to fill a 3 inch pie tin.
    4. Cure the sample in a pie tin or similar container for at least 48 hours.
    5. Treat half of each sample with appropriate surface finishing.
    6. Compare samples to actual stone, and make new samples as necessary to achieve a color match.
  2. Mock-up: Apply a test patch to a small area.
    1. Check to see that the composite patch matches the stone in color, texture and surface treatment.
    2. See that the patch adheres well to the adjacent stone and does not shrink, crack or fall away.
    3. See that the composite patch does not cause deterioration of the old stone by differing too greatly in hardness, moisture transmission, or thermal expansion and contraction.



  1. Thoro System Products


  1. Cement: Portland cement ASTM C 150, Type II, white.
    NOTE: DO NOT use gray cement; It is more difficult to color and work, shrinks more in curing, and may cause staining.
  2. Lime: ASTM C 207, Type S, high plasticity: Increases cohesion during mixing, slows down the rate of cure, and moderates the qualities which could cause an excessively strong and moisture-resistant cement repair to fail and damage old stone.
  3. Sand:
    1. Local natural sand, graded or masonry mortar conforming to ASTM C 144.
    2. Sand color, size, and texture should match the original as closely as possible to provide the proper visual characteristics without other additives. A sample of the sand is necessary for comparison to the original, and should be approved by the RHPO before beginning repointing work.
    3. The color of the sand shall be the primary factor used to make mortars which match existing adjacent fabrics.
  4. Crushed Sandstone:
    1. Best repairs contain actual sandstone; Use stone removed from the area to be repaired, or other old stone with the same qualities.
    2. Grind it fine enough to pass through a 16-mesh screen, and wash thoroughly.
  5. Dry Pigments:
    1. Use when available crushed stone is not sufficient to give a color match.
    2. Use stable fade-proof mineral oxide pigments either natural- or synthetic-fade.

      NOTE: DO NOT exceed recommended manufacturer's suggested maximum amounts; Too much pigment reduces strength and gives unstable color. Maximum pigment/cement ratio to be 1/10 (verify with manufacturer).
  6. Clean, potable water
  7. Additives:
    1. ACRYL-60 (Thoro System Products), or approved equal: Use only latex admixtures that are labeled nonreemulsifiable like ACRYL-60; Do not use bonding agents that may break down in the presence of moisture.

  8. Hydrochloric Acid:
    NOTE: Chemical products are sometimes sold under a common name. This usually means that the substance is not as pure as the same chemical sold under its chemical name. The grade of purity of common name substances, however, is usually adequate for stain removal work, and these products should be purchased when available, as they tend to be less expensive. Common names are indicated below by an asterisk (*).
    1. A strong corrosive irritating acid.
    2. Other chemical or common names include Chlorhydric acid; Hydrogen chloride; Muriatic acid* (generally available in 18 degree and 20 degree Baume solutions); Marine acid*; Spirit of salt*; Spirit of sea salt*.
    4. Available from chemical supply house, drugstore or pharmaceutical supply distributor, or hardware store.


  1. Trowels
  2. Hawks
  3. Stiff natural bristle brushes
  4. Hammer and cold chisel
  5. Wood screeds

2.04 MIXES

  1. Slurry Coat:
    1. 1 part white Portland cement
    2. 2 parts Type S lime
    3. 6 parts sand
    4. Mix with water and ACRYL-60 in 3:1 ratio
  2. . Scratch Coat:
    1. 1 part white Portland cement
    2. 1 part Type S lime
    3. 6 parts sand
    4. Mix with water and ACRYL-60 in 5:1 ratio
  3. Finish Coat:
    1. 1 part white Portland cement
    2. 1 part Type S lime
    3. 2-3 parts sand
    4. 3-4 parts crushed sandstone
    5. Dry pigments (maximum 10% by weight)
    6. Mix with water and ACRYL-60 (or equivalent) in 5:1 ratio



  1. Deterioration of sandstone due to moisture is evident as spalling, erosion, cracking, flaking and deteriorated mortar joints.
  2. . Before proceeding with any type of repair, examine the sandstone to determine the extent and the cause of the damage. Compare undamaged stone with areas of suspected decay. Use a magnifying glass if necessary. Look closely at the following:
    1. Color: What color is the stone? Is there variation in color within individual stones? Is there variation between stones?
    2. Pattern: Are there swirls, bands, or veins of color within the individual stones?
    3. Texture: Is the stone surface rough or smooth? Is it hard or crumbly? Is the texture uniform or varied?
    4. Surface Tooling: Is the face of the stone rough or smooth? Are there any chiseled grooves? Are there any decorative surface patterns? Are any parts damaged or missing?
    5. Sand Grains: Is the grain size large or small? Are the grain shapes regular or irregular, uniform or varied? Does the grain structure appear densely or loosely packed together? Are there mica flakes present in the stone (these will often appear to glitter on the surface)?
    6. Cementing Material: What color is the material between the grains? Do the grains project from the stone surface, giving the surface a rough texture?
    7. Decay and Old Repairs: Is there evidence of erosion, crumbling, spalling or other types of deterioration? Is there evidence of previous patching or repairs?


  1. Cut or chip out all loose stone with a hammer and cold chisel to a minimum thickness of 1/2"; Undercut the stone so the patch will lock firmly.
  2. Drill holes approximately 1/2 inch deep by 1/4 inch in diameter at varying angles about 2 inches apart along the newly exposed surface.
  3. Remove stone dust from the patch area with bristle brushes and lightly spray the area with water.
  4. Apply a thin slurry coat of approximately 1 part white Portland cement, 2 parts lime and 6 parts sand and any additives as required. Final mix will depend on field testing of mix to get correct color and texture match.
  5. Build the scratch coat layers up to within 3/16 inch of the surface; Each layer should be no less than 3/4 inch and no more than 3 inches thick. Do not feather the edges.
  6. Use a trowel to gouge many scratches into the surface of each layer in order to provide keying; Allow 2-4 hours for each coat to cure, but apply each layer while the previous layer is still damp.
  7. Use wood screeds set in adjacent mortar joints to prevent repairs from extending continuously between separate blocks of stone and remove when the mortar is partly set; Repoint the joint after the patch has cured.
  8. Trowel on a final coat of brownstone stucco.
  9. Work a straight edge back and forth across the width of the patch to strike it off flush.
  10. Execute resurfacing carefully. Finish the surface repair by one of the following:
    1. Acid etching: After the surface has cured 48 hours, brush on Technical Grade hydrochloric acid, diluted 1:5 with water; Rinse the surface thoroughly with clean, clear water.
    2. Rubbing stones: Coarse or fine grade (grits #60, 80, 100, 120); Use dry or with water to hone the surface of well cured repairs.
    3. Stipple with a damp sponge or dry-towel with a wooden float.
    4. Score partially cured repair with stone tools to match original tool marks and patterns.