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Historic Preservation - Technical Procedures

Spectitle:

Removing Dirt Build-Up On Sandstone

Procedure code:

0447003R

Source:

National Capitol Region Specifications

Division:

Masonry

Section:

Sandstone

Last Modified:

11/02/2012

Details:

Removing Dirt Build-Up On Sandstone



REMOVING DIRT BUILD-UP ON SANDSTONE


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on cleaning selected
         areas of dirt-build-up on sandstone by washing with an
         alkaline cleaner (for calcareous sandstone), or by
         washing with a hydrofluoric acid-based cleaner (for most
         other sandstones).  GENERALLY, THIS WORK SHOULD BE
         PERFORMED BY AN EXPERIENCED CONTRACTOR.

    B.  For precautions relating to pressure washing, see 04510-
         04-S.

    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).

1.02 QUALITY ASSURANCE

    A.   Qualifications:  Minimum 10 years experience in historic
         masonry cleaning.

    B.   Reference Standards:

         1.   Secretary of the Interior's "Standards for
              Rehabilitation" (see 01091-04-S).

         2.   Preservation Brief No. 1, "The Cleaning and
              Waterproofing of Historic Masonry Buildings" (see
              04510-06-S).

    C.   Test Panels:  Each alternative method for each type of
         cleaning.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.  ProSoCo, Inc.
         www.prosoco.com
         
2.02 MATERIALS

    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 (*).

    A.   For Calcareous Sandstone:  Alkaline cleaners and water
         rinse, such as "Sure-Klean 766 Limestone & Masonry Pre-Wash/After-wash"
         (ProSoCo, Inc.), or approved equal.  Suitable alkaline
         cleaners will contain potassium hydroxide or aluminum
         hydroxide.

    B.   For Most Sandstones, Excluding Calcareous:  Hydrofluoric
         acid-based cleaner containing not more than 5%
         hydrofluoric acid such as "Sure-Klean Restoration
         Cleaner" (ProSoCo, Inc.), or approved equal.

    C.   Acetic Acid:

         1.   A colorless pungent liquid acid that is the chief
              acid of vinegar and that is used especially in
              synthesis (as of plastics).

         2.   Other chemical or common names include Acetates*;
              Acetous salts*; Salts or esters of acetic acid*Vinegar acid*.  
              (Vinegar itself, which contains about 4% acetic acid, may
              be suitable for some purposes requiring acetic acid.)

         3.   Potential hazards:  CORROSIVE TO FLESH AND
              CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.

         4.   Available from chemical supply house (both
              commercial and scientific), drugstore or
              pharmaceutical supply distributor, grocery store or
              supermarket, or hardware store.

    D.   Plastic sheeting

    E.   Clean, potable water

2.02 EQUIPMENT

    A.   Pressure water rinsing equipment (measuring between 100
         and 400 psi for low-pressure; between 400 and 800 psi for
         medium pressure).

    B.   Fan-type spray tips (15 degree fan spray)

    C.   Stiff fiber-bristle brushes

    D.   Plastic spatula

    E.   Masking tape


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Examine site conditions to determine that current
         drainage is sufficient for adequately and safely removing
         cleaning waste and run-off from the site.

    B.   Test clean a small, inconspicuous area to check for
         adverse effects and damage to the material.

3.02 PREPARATION

    A.   Protection:

         1.   Protect surrounding materials on the site and
              adjacent building surfaces and building landscaping from coming in contact
              with the cleaning materials and run-off.  Cover
              with acid-resistant coatings when using acid-based
              cleaners.  Hydrofluoric acid can SEVERELY etch
              aluminum and glass.

         2.   Provide workers with necessary protection against
              cleaning chemicals, overspray and run-off.

         3.   Prevent cleaning chemicals from coming in contact
              with any painted, polished or metallic surfaces.  

         4.   Divert flow of run-off to drains in compliance with
              municipal codes.  Comply with municipal codes
              regarding containment and disposal of cleaning
              materials.

    B.   Surface Preparation:  

         1.   Before proceeding with cleaning operations, remove
              all miscellaneous hardware, anchors and bird
              excrement from the surface to prevent any
              discoloration.

              NOTE:  BIRD EXCREMENT IN CONTACT WITH CLEANING
              SOLUTION WILL LEAVE A PERMANENT DARK-COLORED STAIN
              ON THE SURFACE.

         2.   Check for open holes and joints in surface and
              repaint or caulk as required to prevent water and
              cleaning solutions from penetrating deeply into the
              wall.

3.03 ERECTION, INSTALLATION, APPLICATION

    NOTE:  WHEN CLEANING, AVOID OVERCLEANING.  AIM FOR ACHIEVING
    85% CLEAN.  MOST DAMAGE OCCURS WHEN ATTEMPTING TO CLEAN THE
    LAST 15%.

    NOTE:  TEST CLEAN A SMALL AREA TO DETERMINE EFFECTIVENESS OF
    CLEANING METHODS, MATERIALS, EQUIPMENT AND WORKING PRESSURES
    SELECTED BEFORE PROCEEDING WITH CLEANING OPERATIONS ON LARGER
    AREAS.  ADJUST METHODS, MATERIALS, EQUIPMENT, PRESSURES, ETC. AS 

   NECESSARY.  ALLOW TIME FOR TEST AREA TO COMPLETELY DRY TO ASSESS

  CLEANING RESULTS AND TO DETERMINE IFTHERE ARE ANY MATERIAL

  CHANGES THAT REQUIRE A MODIFICATION OF THE TREATMENT.  DO NOT PROCEED

  UNTIL AN ACCEPTABLE CLEANING OPERATION HAS BEEN APPROVED AND FULLY DOCUMENTED.

    A.   Pre-wet stone surface using a low pressure wash (between
         100 and 400 psi).

    B.   Loosen dirt by scrubbing the area using a non-metallic
         brush.

    C.  Flood the surface with medium-high water pressure (400-
         600 psi)at a rate of three to four gallons per minute.
         Rinse from top to bottom. KEEP THE STONE SURFACE MOIST
         DURING THE ENTIRE CLEANING PROCESS IN ORDER TO AVOID THE
         FORMATION OF RESIDUAL SALTS ON THE SURFACE.

    D.   FOR CALCAREOUS SANDSTONE, apply a commercial alkaline
         cleaner.  Follow manufacturer's application instructions.

         CAUTION: DO NOT USE ACID-BASED CLEANERS ON CALCAREOUS
         SANDSTONE.  ACIDIC PRODUCTS CAN ETCH OR ABRADE THE STONE.
         ALKALINE CLEANERS SHOULD ALWAYS BE USED ON ACID-SENSITIVE
         MASONRY.  AVOID USING ALKALINE SOLUTIONS CONTAINING
         SODIUM HYDROXIDE (CAUSTIC SODA OR LYE) OR AMMONIUM
         BIFLUORIDE.  THESE HAVE THE POTENTIAL TO CAUSE
         EFFLORESCENCE, SUBFLORESCENCE AND CAN LEAD TO ABRASION OF
         THE SURFACE.

         1.   Apply alkaline cleaner pre-wash using a soft nylon
              bristle brush.  Allow to remain on the surface for
              30 minutes, or as long as determined by testing.

         2.   Rinse the surface thoroughly with clean, clear
              water using pressure between 400 and 600 psi. 

             Direct the spray downward to avoid forcing water into joints and the stone surface.
              Monitor interior spaces at cleaning locations and contain water from rinsing operation so as not to
              flood building interior.

         3.   While surface is still wet from rinsing off pre-
              wash, apply limestone afterwash with a soft, fiber
              bristled brush.  

              a.   Dilute limestone afterwash with as much water
                   as determined effective during testing.

              b.   If surface has begun to dry, re-wet before
                   applying afterwash.

              c.   Cover all corners, moldings, and interstices
                   of the limestone.

         4.   Allow to stand for 3-5 minutes or as long as
              determined by testing.

         5.   Rinse the surface thoroughly with clean, clear
              water using pressure between 400 and 600 psi. Direct spray downward as noted above.
              Monitor interior spaces and contain water from rinsing operation so as not to
              flood building interior.
         
         6.   Spray on an application of Acetic Acid in order to
              neutralize the alkaline wash.

         7.   Rinse the stone with clean, clear water again.

         8.   For very light calcareous sandstone, the use of a
              complexing agent such as EDTA or organic bleach may
              be required in order to achieve uniform results.
              USE ONLY UNDER PROFESSIONAL SUPERVISION.

    E.   For Most Sandstones, (EXCLUDING CALCAREOUS SANDSTONE),
         apply a commercial hydrofluoric acid-based cleaner.
         Follow manufacturer's application instructions.

         CAUTION: HYDROFLUORIC ACID MAY LIGHTEN THE COLOR OF SOME
         SANDSTONES CONTAINING IRON.  DO A TEST PATCH IN NOT HIGHLY VISIBLE AREA, MONITOR RESULTS, AND ACT                                ACCORDINGLY.

         1.   Apply acid-based cleaner using a soft nylon bristle
              brush.  Allow to remain on the surface as long as
              determined by testing.  DO NOT EXCEED 5-7 MINUTES.

         2.   Rinse the surface thoroughly with clean, clear
              water using pressure between 400 and 600 psi.
              Direct spray downward as noted above. Monitor interior spaces and contain water from rinsing operation so as not to
              flood building interior.

         3.   If efflorescence appears, clean a second time with
              a hydrofluoric acid-based cleaner, followed
              immediately by rinsing with cold water.

                         END OF SECTION
 


calcareous sandstone, sandstone, alkaline cleaner,hydrofluoric acid-based cleaner, dirt, dirt stains, dirt build-up