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Historic Preservation - Technical Procedures

Spectitle:

Supplemental Guidelines For Removing Paint From Interior And Exterior Wood Surfaces

Procedure code:

0640002S

Source:

Hspg Prepared For Nps - Sero

Division:

Wood And Plastics

Section:

Architectural Woodwork

Last Modified:

02/24/2012

Details:

Supplemental Guidelines For Removing Paint From Interior And Exterior Wood Surfaces



SUPPLEMENTAL GUIDELINES FOR REMOVING PAINT FROM INTERIOR AND
EXTERIOR WOOD SURFACES


This standard identifies the causes of paint failure on wood
surfaces and provides basic guidelines for deciding to what extent
deteriorated paint layers should be removed.  This procedure should
be used in conjunction with 06400-07-R, "Chemically Removing Paint
from Wood Features", and 06400-09-R, "Removing Paint from Wood
Features Using Thermal Methods".


***GENERAL***

-    Exterior surfaces are painted both for aesthetics and for
    protection.  Paint protects the wood substrate from
    ultraviolet degradation due to sunlight exposure and rotting
    due to excess moisture.

-    Interior wood surfaces are usually painted for decorative
    reasons rather than for protection.  

-    Causes for premature paint failure:

    1.   Excess moisture in wood causes the wood to swell,
         breaking the bond between the wood and the paint.

    2.   Poor surface preparation interferes with the bond between
         the new paint layer(s) and the substrate.  

    3.   The wrong type of paint used in the wrong way and/or in
         the wrong place.


***TYPES OF PAINT FAILURE***

PEELING/FLAKING:  

-    Paint may peel for a number of reasons:

    1.   When applied over damp wood, (usually only a problem when
         water blasting has been used to remove loose paint)

    2.   If painting was begun too soon after heavy rains.  

         NOTE:  USE A MOISTURE METER TO DETERMINE THE AMOUNT OF
         MOISTURE IN THE WOOD.  MAXIMUM MOISTURE CONTENT IS 14%.

    3.   When excessive moisture inside the wall migrates to the
         outside.  The moisture may come from poorly vented
         bathrooms, kitchens, and laundries, or leaky gutters and
         flashing, or broken plumbing.  

    4.   When applied to a dirty or greasy surface.  The paint
         will not adhere and will cause intercoat peeling.  The
         new paint film will simply peel off leaving the bottom
         paint layers intact.  This is especially a problem:

         -    under roof eaves and other protected areas not
              readily washed by rain.  

         -    when a slick surface is painted without first
              sanding it.

         -    when an incompatible top coat is used.  

         -    when the top coat is applied more than two weeks
              after the surface was painted with an oil-based
              primer.  A soap-like material forms on the surface
              of the primer which needs to scrubbed off with
              detergent and water before the top coat is applied.
              If the surface is not scrubbed clean, the top coat
              will peel.

    5.   If the existing thickness of paint layers has reached or
         exceeded 16 mils and additional layers of paint have been
         added.  Paint film thickness at 16 mils or more is said
         to have reached its saturation point.  Additional layers
         of paint cause peeling for a number of reasons:

         -    The thick paint layers are less permeable to water
              vapor.  Since the moisture cannot evaporate,
              pressure builds up behind the paint and peeling or
              blisters result.  

         -    The individual layers of paint can no longer expand
              and contract at the same rate and the older, more
              brittle layers fail resulting in peeling and
              cracking.  

    6.   When exterior wooden elements have exposed end grain.
         Water absorbed in these areas causes the wood to swell,
         which loosens the bond between the wood and the paint.  

         -    Susceptible areas include the ends of clapboard
              where they meet door and window trim or corner
              boards, butt and miter joints of clapboard and
              other trim pieces, and porch floor boards.  

    7.   When water becomes trapped inside exterior hollow wooden
         elements such as columns or built-up fence newels, and
         adequate ventilation is not provided.  Water vapor
         trapped inside can condense and settle at the base of the
         element, creating ideal conditions for rot.

    8.   When the surface has not been adequately washed.  This is
         especially a problem if latex paint is applied over
         calcimine paint which is water soluble.

    9.   When protected areas are not readily washed by rain,
         causing dirt to accumulate on the surface.  The dirt may
         have a tendency to attract and hold moisture against the
         building.  

         -    The prolonged presence of moisture, combined with
              the lack of sunlight, can cause the top layer of
              paint to expand and contract more frequently than
              the lower layers, often resulting in a breaking of
              the bond between the paint layers and the wood
              substrate.

         -    Protected areas to watch include eaves, soffits,
              tops of walls, or areas protected by trees and
              other vegetation.  

    10.  If the species of wood used in construction is not suited
         dimensionally to provide the least amount of stress on
         the paint film, given the expansion and contraction rates
         associated with normal changes in relative humidity.  For
         example, edge-grain, or quarter-sawn, softwoods are more
         dimensionally stable than flat sawn boards, warping and
         shrinking less.  This places less stress on the paint
         film, thereby reducing the likelihood of cracking and
         peeling.

BLISTERS:

-    Blisters may occur for several reasons:

    1.   If the paint was applied in direct sunlight.  The paint
         film forms a skin before the thinners of the paint have
         had a chance to evaporate and a blister forms.  Usually
         a sound layer of paint is visible when the blister is
         split open.  

    2.   When paint has reached its saturation point as described
         above, or when paint has been applied to a wet surface.
         Usually bare wood is visible when the blister is split
         open.

    3.   If a primer containing zinc oxide, or a finish coat
         containing zinc oxide without a proper prime coat is
         used.  Zinc oxide is hydrophilic, meaning it has a strong
         affinity for water and will readily absorb moisture.

CRAZING AND CRACKING:

-    Crazing and cracking usually occur:

    1.   When old, thick layers of paint can no longer expand and
         contract at the same rate as the wood substrate.
         Initially, only the top layers are affected.  However, as
         water gets into these fine, hairline cracks, they
         eventually deepen and widen to form major cracks.  

ALLIGATORING:

-    Alligatoring is an advanced stage of cracking where the
    deteriorated paint film takes on the appearance of alligator
    skin.  It may occur:

    1.   When a top coat is applied over a glossy paint surface
         that has not first been roughened to provide a proper
         "tooth" for the new paint film.

WRINKLING:

-    Wrinkling is when the top layer of paint moves, or dries,
    while the paint underneath is also still drying, and also
    still moving, but at a different rate.  This may occur:

    1.   When the top coat is applied too thickly or not fully
         brushed out, allowing the top of the paint film to dry
         before the bottom of the film dries.

    2.   When the second coat is applied before the first coat has
         had a chance to dry.

    3.   If the paint is applied in hotter weather than the
         manufacturer recommends.  High temperatures cause the top
         of the paint film to dry too quickly, before the bottom
         of the film has had a chance to dry.

MILDEW:

-    Mildew is likely to occur:

    1.   On damp paint films.

    2.   On crazed, cracked or peeling paint surfaces.  Paint
         layers that are crazed and cracked are especially prone
         to mildew growth because moisture concentrates in the
         cracks.  

         Note:  Painting over mildew without first killing it will
         not solve the problem.  Mildew will just grow through the
         new paint.  A sunny South or West facade is no guarantee
         that mildew will not grow.  


***DECIDING WHEN AND HOW MUCH PAINT TO REMOVE***

GENERAL:  It is important when making the decision to remove paint
to determine why the paint is to be removed, because to do so is a
time consuming and expensive job.  (If the decision is made to
remove all of the paint, samples of the existing paint layers
should be taken to document and identify the paint colors used
throughout the history of the building.  A section of the existing
paint film, located in an inconspicuous area, should be left alone
and covered to allow for future study.)

-    Paint should be removed when it has built up to the point of
    obscuring decorative details.

-    Selective paint removal is also often done to expose a
    previous decorative finish such as graining or stenciling, or
    to restore a varnished or shellacked finish.

-    The appropriate finish should be consistent with the original
    finish treatment.  To expose the "natural beauty" of the wood,
    if the wood has always been painted is to impose modern tastes
    in interior decoration and should be avoided.

PEELING/FLAKING:  

-    For wholesale peeling and/or paint which has reached its
    saturation point:  

    1.   Remove all of the paint before repainting.  

-    For localized paint failure:

    1.   Remove only the affected layers of paint.

    2.   Sand the edges of the sound paint to provide a smooth
         transition between the old and the new

    3.   Spot prime the area and repaint as required and as
         described in procedure 06300-01-S, 06300-02-R and 09900-07-S.

BLISTERS:

-    For solvent blisters, or those where sound layers of paint are
    still visible under the blister:

    1.   Remove only the failed layers of paint.  It is usually
         not necessary to remove paint to the bare wood.

    2.   Spot prime and repaint as required and as described in
         procedure 06300-01-S, 06300-02-R and 09900-07-S.

-    For localized water blisters:

    1.   Treat as for solvent blisters above if the surrounding
         paint is sound.  

-    For localized water blisters in conjunction with massive
    peeling of thick layers of paint:

    1.   Remove all of the paint.

    2.   Prime and repaint as required and as described in
         procedure 06300-01-S, 06300-02-R and 09900-07-S.

CRACKING AND CRAZING:

-    For surface crazing:

    1.   Sand the paint film only as necessary to remove the
         crazed layers of paint.  

    2.   Repainting may or may not be necessary.  

-    For cracking that reveals bare wood or a dark varnished or
    shellacked surface:

    1.   Completely remove all paint.

    2.   Prime and repaint as required and as described in
         procedure 06300-01-S, 06300-02-R and 09900-07-S.

WRINKLING:

-    For wrinkles in paint surfaces:

    1.   Sand the surface to the next unwrinkled layer.

    2.   Repaint as required and as described in procedure 06300-01-S,
         06300-02-R and 09900-07-S.

ALLIGATORED:

-    For paint that has alligatored to form deep cracks:

    1.   Completely remove all of the paint.

    2.   Prime and repaint as required and as described in
         procedure 06300-01-S, 06300-02-R and 09900-07-S.

MILDEW:

-    For mildew growth:  

    1.   Wash with a solution of bleach to kill the mildew.  If
         the surface is also dirty, adding TSP to the bleach
         solution will aid in the cleaning process.

-    For mildew associated with cracks in the paint film or other
    type of paint deterioration:

    1.   Treat the paint film as directed above for complete paint
         removal and repaint as required and as described in
                   procedure 06300-01-S, 06300-02-R and 09900-07-S.


***PAINT REMOVAL TECHNIQUES***

-    Paint removal is achieved through a variety of means:

    1.   Thermal methods, such as heat plates and heat guns; See
         procedure 06400-09-R, "Removing Paint from Wood Features
         Using Thermal Methods" for guidance.

    2.   Abrasive methods, such as by hand or with an orbital
         sander; See procedure 06300-02-R, "Procedures for
         Painting Wood Features" for guidance.

    3.   Chemical methods; See procedure 06400-07-R, "Chemically
         Removing Paint from Wood Features" for guidance.

-    Applications of the above methods should be reviewed in
    accordance with the Secretary of the Interior's "Standards for
    Rehabilitation Projects."

                         END OF SECTION