Patching Weathered, Exfoliated, Or Blistering Sandstone

Procedure code:
Developed For Hspg (Nps - Sero)
Last Modified:




    A.   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

    B.   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.

    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 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).

    D.   For general information on the characteristics, uses and
         problems associated with sandstone, see 04470-01-S.


    A.   American Society for Testing and Materials (ASTM)

    A.   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

         6.   Compare samples to actual stone, and make new
              samples as necessary to achieve a color match.

    B.   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.



    A.  Thoro System Products

    A.   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

    B.   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.

    C.   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

    D.   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.

    E.   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).

    F.   Clean, potable water

    G.   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


    H.   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*.

         3.   Potential Hazards:  TOXIC, CORROSIVE TO FLESH;

         4.   Available from chemical supply house, drugstore or
              pharmaceutical supply distributor, or hardware


    A.   Trowels

    B.   Hawks

    C.   Stiff natural bristle brushes

    D.   Hammer and cold chisel

    E.   Wood screeds

2.04 MIXES

    A.   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

    B.   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

    C.   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



    A.   Deterioration of sandstone due to moisture is evident as
         spalling, erosion, cracking, flaking and deteriorated
         mortar joints.

    B.   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

         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?


    A.   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.

    B.   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.

    C.   Remove stone dust from the patch area with bristle
         brushes and lightly spray the area with water.

    D.   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.

    E.   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

    F.   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.

    G.   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.

    H.   Trowel on a final coat of brownstone stucco.

    I.   Work a straight edge back and forth across the width of
         the patch to strike it off flush.

    J.   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.

                         END OF SECTION

Last Reviewed 2012-08-09