Applying a Water-Repellent Preservative to Wood

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 specification provides guidance on applying a water-repellent preservative (WRP) to wood. This coating will prolong the service life of wood and provide some protection against agents of deterioration.


  3. Natural causes of deterioration include decay, ultraviolet degradation, insect infestation and excess moisture.

  4. WRPs are often recommended for humid climates. Their use can significantly reduce the problems of peeling, flaking, blistering, etc. of painted wood surfaces.

  5. Some types of problems resulting from the weathering process include:

    1. Fungi and/or mildew growth.

    2. Warped boards.

    3. Loose fasteners.

    4. Changes in surface texture resulting in cracks and checks.

  6. In addition to opaque paints, various so-called "natural" finishes and colored stains provide WRP protection.

  7. Like when preparing paint, proper surface preparation and application are vital to long-lasting protection.

  8. Read "General Project Guidelines" along with this specification. 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). The 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)


  1. Water-Repellent Preservatives (WRPs):

    1. Natural, colorless liquids which, when brushed onto or soaked into the wood, render the wood impervious to liquid water, inhibit the growth of mildew and other fungi, and provide protection against termite and other insect infestation.

    2. Their use reduces warping and checking and prevents water staining at edges of boards and at the end grain.

    3. WRPs do not, however, protect wood from water vapor or ultraviolet degradation.

    4. WRPs contain no coloring agents but their application will cause wood to darken somewhat. The resulting color is typically a golden tan, although the color will vary according to the type of wood.

    5. WRPs can be used as a natural finish.

  2. Water Repellents

    1. Similar in composition and function to WRPs, except that water repellents do not contain a fungicide or mildewcide.

    2. Additionally, water repellents do not provide adequate protection against decay and ultraviolet degradation. Therefore, they should not to be used as the only finish.

    3. NOTE: The appearance of the phrase "mildew-resistant" on a water repellent does not mean the product is a preservative. Therefore, read the label carefully when purchasing a WRP.


Environmental Requirements:

  1. Unless otherwise recommended by the manufacturer, the ambient temperature shall be between 50 and 95 degrees Fahrenheit when applying either a stain or WRP.

  2. Do not apply a WRP when the relative humidity exceeds 85% or the moisture content of the wood exceeds 12%, as measured by an electronic moisture meter.

  3. Do not apply a WRP in the direct sun. They shall be applied only when the surface to be treated is in the shade and the sun is shining on the opposite elevation:

    1. The west elevation should be treated in the morning when the sun is shining on the east elevation.

    2. The north elevation should be treated around noon when the sun is shining on the south elevation

    3. The east elevation should be treated in the afternoon when the sun is shining on the west elevation.

    4. The south elevation should be treated in the late afternoon when it is in full shade.

  4. Do not apply WRPs to damp surfaces, in misty or rainy weather, in the snow or where there is visible ice or frost on the surfaces.


  1. On smooth wood surfaces, a WRP will remain effective for about a year.

    1. If the first application was applied to the point of refusal (total saturation), it may remain effective for two years.

    2. On rough or weathered wood, expect a WRP to remain effective from one to three years.

  2. To determine if a WRP finish is still effective, splash some water on the surface.

    1. If the water beads up, the WRP is still providing the necessary protection.

    2. If the water soaks into the wood and/or the wood has a blotchy appearance (caused by mildew), it is necessary to retreat.

  3. Before applying a new coat of WRP, clean the old surface with a nonferrous bristle brush.

  4. To kill any mildew, wash with a solution of 1/3 cup of household detergent (NO AMMONIA), 1 quart 5% bleach, and 3 quarts warm water.

  5. Rinse well and let dry thoroughly before reapplying the WRP.

  6. After the treated wood has achieved a uniform tan color, retreatment will be required every 2 to 4 years.



  1. American Building Restoration Products, Inc.

Franklin, WI


  1. Rust-Oleum, woodcare products division



NOTE: It is very important to be aware of how different manufacturers' repellents react to being top-coated with paints of varying chemical compositions (latex/oil/alkyd/etc.). Be sure to fully investigate manufacturer's datasheets and application guidelines before moving forward with any work.

  1. WRP

    1. Use a commercial WRP such as "X-100 Natural Seal Stain" (ABR Products, Inc.), "Wolman F&P Finish and Preservative" (Rust-Oleum), "Wolman Woodlife Classic Clear Wood Preservative" (Rust-Oleum) or approved equal.

    1. -OR-

    1. Prepare home-made WRP based on the USDA Forest Products Laboratory formula (see "Preparing a Non-Toxic Water-Repellent Preservative" for guidance on preparation).

    2. For warranty purposes, using a commercial product is often preferable.

  2. Household detergent (NO AMMONIA).

  3. Household Bleach:

    1. Other chemical or common names include sodium hypochlorite (NaOCl), bleaching solution; laundry bleach, and solution of chlorinated soda.

    2. Potential Hazards: CAUSTIC TO FLESH.

    3. Available from chemical supply house, grocery store, supermarket, hardware store or janitorial supply distributor.

  4. Clean, clear water.



  1. Use natural bristle paint brushes for oil/alkyd preservatives. Precondition brushes by soaking them in raw linseed oil for 24 hours.

  2. Use nylon bristle brushes for applying latex water-based preservatives.

  3. Do not use the same brush for both types of stain.

  4. For thin, runny stains, foam pad applicators can be used.

  5. Stiff natural bristle scrub brushes.



Surface Preparation: The surface should be free of all loose fibers, dust and grease before application of a WRP.


  1. Dipping is the most effective means of treatment, especially for the ends of wood members. Brushing to the point of refusal (total saturation) is the next best method of treatment.

    1. For treated lumber, dip freshly cut surfaces before installation, 10 seconds to 3 minutes.

    2. For untreated lumber, dip, brush or spray with preservative. Pay particular attention to end grain and joints.

    3. For wood shingles, dip before installation, with a second coat brushed onto the surface after installation.

    4. On fixed surfaces, use a minimum of two successive coats.

    5. For pieces that are removable, soak for 10 seconds to 3 minutes.

  1. NOTE: On a smooth surface, 1 gallon of WRP will cover approximately 250 square feet, depending on the manufacturer. On a rough surface, it will cover only 100-150 square feet.

  2. Allow adequate time for WRP treatment to dry before repainting so that paint will adhere properly. Always follow the manufacturer's instructions.

    1. In general, if the surfaces have been brush-treated, a drying time of 48 hours at 70 degrees Fahrenheit is generally sufficient.

    2. Longer drying times will be required if ambient temperatures are below 70 degrees Fahrenheit at any time during this drying period.

    3. Wood that has been dipped for 10 seconds will need a minimum of one week of similar, ideal drying time.

    4. If is too cold in the evenings for any paint film to dry properly (i.e. if it falls below 50 degrees Fahrenheit overnight), only apply a WRP at this time, and wait for warmer weather (even if this means waiting for springtime) to prime and paint over it.

  3. In addition to adequate drying times, some wood surfaces treated with water repellents/WRPs must be allowed time to weather before they can be painted. Follow the manufacturer's instructions, as the weathering time can vary from six months to two years.


Caulking joints is an important part of surface preparation. You should generally caulk after a WRP or water repellent has been applied. However, check the manufacturer's specifications for both the preservative and the caulk being used, to ensure compatibility and attain maximum performance.