U.S. General Services Administration
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.
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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.
PART 1–GENERAL
1.01 SUMMARY
- This procedure includes guidance on how to remove a surface applied waterproof coating which has begun to discolor and flake-off due age, poor penetration or improper application.
- Safety Precautions:
- Workers shall be provided with protective clothing including full head and face protection and noise protection.
- Helmet shall be “air-line” type to maintain positive air pressure inside helmet to prevent the ingress of dangerous abrasive dust.
- 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
- Submittals
- 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).
PART 2–PRODUCTS
2.01 MATERIALS
- Fine abrasive - type to be determined by field testing completed prior to actual cleaning. Possible abrasives include talc, ground nut shells or egg shells, both angular and smooth sands depending on cutting edge required to remove deteriorated coating.
- Water - clean, clear, potable.
2.02 EQUIPMENT
- Air compressor (with pressure gauge)
- Appropriate nozzle - long venturi nozzle for flat surfaces with consistent soiling conditions; short straight nozzles for detailed areas. Shapes to be determined by field testing. Nozzle should be type that allows for different modes of operation, abrasive and air, abrasive and water, air only and water only to allow greater control by operator.
- High pressure water lance (maximum pressure not to exceed 400 psi.)
PART 3–EXECUTION
3.01 ERECTION, INSTALLATION, APPLICATION
- Using low pressures (60 to 80 psi at the compressor), apply stream of abrasive/water mixture to wall surface being cleaned. Begin with lowest pressure and softest abrasive determined appropriate by pretesting.
- Maintain minimum 12” from the surface of the stone and do not allow stream to stay in one spot for more than a few seconds.
- Do not allow direct blasts at mortar joints.
- Check work frequently and stop cleaning as soon as coating has been removed.
- Reduce air pressure if damage to stone occurs.
- To avoid “gun-shading” do not rush through work, and maintain steady, even control.
- After surfaces have been cleaned, wash surfaces to remove all traces of slurry and spent abrasives. Use a low-volume, high-pressure water lance, with pressure not to exceed 400 psi at the compressor. Use hand-sprays as necessary on small scale details.