Preservation Briefs: 10 Exterior Paint Problems On Historic Woodwork

Procedure code:
Preservation Briefs 10, National Park Service, Pad
Exterior Painting
Last Modified:




The link immediately below connects to the latest version of National Park Service Preservation Brief 10:

Kay D. Weeks and David W. Look, AIA

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
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This Brief expands on that advice for the architect, building
manager, contractor, or homeowner by identifying and describing
common types of paint surface conditions and failures, then
recommending appropriate treatments for preparing exterior wood
surfaces for repainting to assure the best adhesion and greatest
durability of the new paint.  Although the Brief focuses on
responsible methods of "paint removal," several paint surface
conditions will be described which do not require any paint
removal, and still others which can be successfully handled by
limited paint removal.  In all cases, the information is intended
to address the concerns related to exterior wood.  It will also be
generally assumed that, because houses built before 1950 involve
one or more layers of lead-base paint, the majority of conditions
warranting paint removal will mean dealing with this toxic
substance along with the dangers of the paint removal tools and
chemical strippers themselves.


Paint applied to exterior wood must withstand yearly extremes of
both temperature and humidity.  While never expected to be more
than a temporary physical shield--requiring reapplication every 5-8
years--its importance should not be minimized.  Because one of the
main causes of wood deterioration is moisture penetration, a
primary purpose for painting wood is to exclude such moisture,
thereby slowing deterioration not only of a building's exterior
siding and decorative features but, ultimately, its underlying
structural members.  Another important purpose for painting wood
is, of course, to define and accent architectural features and to
improve appearance.


Exterior paint is constantly deteriorating through the processes of
weathering, but in a program of regular maintenance--assuming all
other building systems are functioning properly--surfaces can be
cleaned, lightly scraped, and hand sanded in preparation for a new
finish coat.  Unfortunately, these are ideal conditions.  More
often, complex maintenance problems are inherited by owners of
historic buildings, including areas of paint that have failed
beyond the point of mere cleaning, scraping, and hand sanding
(although much so-called "paint failure" is attributable to
interior or exterior moisture problems or surface preparation and
application mistakes with previous coats).

Although paint problems are by no means unique to historic
buildings, treating multiple layers of hardened, brittle paint on
complex, ornamental--and possibly fragile--exterior wood surfaces
necessarily requires an extremely cautious approach.  In the case
of recent construction this level of concern is not needed because
the wood is generally less detailed and, in addition, retention of
the sequence of paint layers as a partial record of the building's
history is not an issue.

When historic buildings are involved, however, a special set of
problems arises--varying in complexity depending upon their age,
architectural style, historical importance, and physical soundness
of the wood--which must be carefully evaluated so that decisions
can be made that are sensitive to the longevity of the resource.


At the outset of this Brief, it must be emphasized that removing
paint from historic buildings--with the exception of cleaning,
light scraping, and hand sanding as part of routine
maintenance--should be avoided unless absolutely essential.  ONCE
speaking, paint can adhere just as effectively to existing paint as
to bare wood, providing the previous coats of paint are also
adhering uniformly and tightly to the wood and the surface is
properly prepared for repainting--cleaned of dirt and chalk and
dulled by sanding.  But, if painted exterior wood surfaces display
continuous patterns of deep cracks or if they are extensively
blistering and peeling so that bare wood is visible, then the old
paint should be completely removed before repainting.  The only
other justification for removing all previous layers of paint is if
doors, shutters, or windows have literally been "painted shut," or
if new wood is being pieced-in adjacent to old painted wood and a
smooth transition is desired.


Because paint removal is a difficult and painstaking process, a
number of costly, regrettable experiences have occurred--and
continue to occur--for both the historic building and the building
owner.  Historic buildings have been set on fire with blow torches;
wood irreversibly scarred by sandblasting or by harsh mechanical
devices such as rotary sanders and rotary wire strippers; and
layers of historic paint inadvertently and unnecessarily removed.
In addition, property owners, using techniques that substitute
speed for safety, have been injured by toxic lead vapors or dust
from the paint they were trying to remove or by misuse of the paint
removers themselves.

Owners of historic properties considering paint removal should also
be aware of the amount of time and labor involved.  While removing
damaged layers of paint from a door or porch railing might be
readily accomplished within a reasonable period of time by one or
two people, removing paint from larger areas of a building can,
without professional assistance, easily become unmanageable and
produce less than satisfactory results.  The amount of work
involved in any paint removal project must therefore be analyzed on
a case-by-case basis.  Hiring qualified professionals will often be
a cost-effective decision due to the expense of materials, the
special equipment required, and the amount of time involved.
Further, paint removal companies experienced in dealing with the
inherent health and safety dangers of paint removal should have
purchased such protective devices as are needed to mitigate any
dangers and should also be aware of State or local environmental
and/or health regulations for hazardous waste disposal.

All in all, paint removal is a messy, expensive, and potentially
dangerous aspect of rehabilitating or restoring historic buildings
and should not be undertaken without careful thought concerning
first, its necessity, and second, which of the available
recommended methods is the safest and most appropriate for the job
at hand.


If existing exterior paint on wood siding, eaves, window sills,
sash, and shutters, doors, and decorative features shows no
evidence of paint deterioration such as chalking, blistering,
peeling, or cracking, then there is no physical reason to repaint,
much less remove paint! Nor is color fading, of itself, sufficient
justification to repaint an historic building.

The decision to repaint may not be based altogether on paint
failure.  Where there is a new owner, or even where ownership has
remained constant through the years, taste in colors often changes.
Therefore, if repainting is primarily to alter a building's primary
and accent colors, a technical factor of paint accumulation should
be taken into consideration.  When paint builds up to a thickness
of approximately 1/16" (approximately 16-30 layers), one or more
extra coats of paint may be enough to trigger cracking and peeling
in limited or even widespread areas of the building's surface.
This results because excessively thick paint is less able to
withstand the shrinkage or pull of an additional coat as it dries
and is also less able to tolerate thermal stresses.  Thick paint
invariably fails at the weakest point of adhesion -- the oldest
layers next to the wood.  Cracking and peeling follow.  Therefore,
if there are no signs of paint failure, it may be somewhat risky to
add still another layer of unneeded paint simply for color's sake
(extreme changes in color may also require more than one coat to
provide proper hiding power and full color).  When paint appears to
be nearing the critical thickness, a change of accent colors (that
is, just to limited portions of the trim) might be an acceptable
compromise without chancing cracking and peeling of paint on wooden

If the decision to repaint is nonetheless made, the "new" color or
colors should, at a minimum, be appropriate to the style and
setting of the building.  On the other hand, where the intent is to
restore or accurately reproduce the colors originally used or those
from a significant period in the building's evolution, they should
be based on the results of a paint analysis.


It is assumed that a preliminary check will already have been made
to determine, first, that the painted exterior surfaces are indeed
wood--and not stucco, metal, or other wood substitutes--and second,
that the wood has not decayed so that repainting would be
superfluous.  For example, if any area of bare wood such as window
sills has been exposed for a long period of time to standing water,
wood rot is a strong possibility.  Repair or replacement of
deteriorated wood should take place before repainting.  After these
two basic issues have been resolved, the surface condition
identification process may commence.

The historic building will undoubtedly exhibit a variety of
exterior paint surface conditions.  For example, paint on the
wooden siding and doors may be adhering firmly; paint on the eaves
peeling; and paint on the porch balusters and window sills cracking
and alligatoring.  The accurate identification of each paint
problem is therefore the first step in planning an appropriate
overall solution.

Paint surface conditions can be grouped according to their relative
severity: CLASS I conditions include minor blemishes or dirt
collection and generally require no paint removal; CLASS II
conditions include failure of the top layer or layers of paint and
generally require limited paint removal; and CLASS III conditions
include substantial or multiple-layer failure and generally require
total paint removal.  It is precisely because conditions will vary
at different points on the building that a careful inspection is
critical.  Each item of painted exterior woodwork (i.e., siding,
doors, windows, eaves, shutters, and decorative elements) should be
examined early in the planning phase and surface conditions noted.



-    Cause of Condition:

    Environmental "grime" or organic matter that tends to cling to
    painted exterior surfaces and, in particular, protected
    surfaces such as eaves, do not constitute a paint problem
    unless painted over rather than removed prior to repainting.
    If not removed, the surface deposits can be a barrier to
    proper adhesion and cause peeling.

-    Recommended Treatment:

    Most surface matter can be loosened by a strong, direct stream
    of water from the nozzle of a garden hose.  Stubborn dirt and
    soot will need to be scrubbed off using 1/2 cup of household
    detergent in a gallon of water with a medium soft bristle
    brush.  The cleaned surface should then be rinsed thoroughly,
    and permitted to dry before further inspection to determine if
    repainting is necessary.  Quite often, cleaning provides a
    satisfactory enough result to postpone repainting.


-    Cause of Condition:

    Mildew is caused by fungi feeding on nutrients contained in
    the paint film or on dirt adhering to any surface.  Because
    moisture is the single most important factor in its growth,
    mildew tends to thrive in areas where dampness and lack of
    sunshine are problems such as window sills, under eaves,
    around gutters and downspouts, on the north side of buildings,
    or in shaded areas near shrubbery.  It may sometimes be
    difficult to distinguish mildew from dirt, but there is a
    simple test to differentiate: if a drop of household bleach is
    placed on the suspected surface, mildew will immediately turn
    white whereas dirt will continue to look like dirt.

-    Recommended Treatment:

    Because mildew can only exist in shady, warm, moist areas,
    attention should be given to altering the environment that is
    conducive to fungal growth.  The area in question may be
    shaded by trees which need to be pruned back to allow sunlight
    to strike the building; or may lack rain gutters or proper
    drainage at the base of the building.  If the shady or moist
    conditions can be altered, the mildew is less likely to
    reappear.  A recommended solution for removing mildew consists
    of one cup non-ammoniated detergent, one quart household
    bleach, and one gallon water.  When the surface is scrubbed
    with this solution using a medium soft brush, the mildew
    should disappear; however, for particularly stubborn spots, an
    additional quart of bleach may be added.  After the area is
    mildew-free, it should then be rinsed with a direct stream of
    water from the nozzle of a garden hose, and permitted to dry
    thoroughly.  When repainting, specially formulated
    "mildew-resistant" primer and finish coats should be used.


-    Cause of Condition:

    Chalking--or powdering of the paint surface--is caused by the
    gradual disintegration of the resin in the paint film.  (The
    amount of chalking is determined both by the formulation of
    the paint and the amount of ultraviolet light to which the
    paint is exposed.)  In moderation, chalking is the ideal way
    for a paint to "age," because the chalk, when rinsed by
    rainwater, carries discoloration and dirt away with it and
    thus provides an ideal surface for repainting.  In excess,
    however, it is not desirable because the chalk can wash down
    onto a surface of a different color beneath the painted area
    and cause streaking as well as rapid disintegration of the
    paint film itself.  Also, if a paint contains too much pigment
    for the amount of binder (as the old white lead carbonate/oil
    paints often did), excessive chalking can result.

-    Recommended Treatment:

    The chalk should be cleaned off with a solution of l/2 cup
    household detergent to one gallon water, using a medium soft
    bristle brush.  After scrubbing to remove the chalk, the
    surface should be rinsed with a direct stream of water from
    the nozzle of a garden hose, allowed to dry thoroughly, (but
    not long enough for the chalking process to recur) and
    repainted, using a non-chalking paint.


-    Cause of Condition:

    Staining of paint coatings usually results from excess
    moisture reacting with materials within the wood substrate.
    There are two common types of staining, neither of which
    requires paint removal.  The most prevalent type of stain is
    due to the oxidation or rusting of iron nails or metal (iron,
    steel, or copper) anchorage devices.  A second type of stain
    is caused by a chemical reaction between moisture and natural
    extractives in certain woods (red cedar or redwood) which
    results in a surface deposit of colored matter.  This is most
    likely to occur in new replacement wood within the first 10-15

-    Recommended Treatment:

    In both cases, the source of the stain should first be located
    and the moisture problem corrected.

    When stains are caused by rusting of the heads of nails used
    to attach shingles or siding to an exterior wall or by rusting
    or oxidizing iron, steel, or copper anchorage devices adjacent
    to a painted surface, the metal objects themselves should be
    hand sanded and coated with a rust inhibitive primer followed
    by two finish coats.  (Exposed nail heads should ideally be
    countersunk, spot primed, and the holes filled with a high
    quality wood filler except where exposure of the nail head was
    part of the original construction system or the wood is too
    fragile to withstand the countersinking procedure.)

    Discoloration due to color extractives in replacement wood can
    usually be cleaned with a solution of equal parts denatured
    alcohol and water.  After the affected area has been rinsed
    and permitted to dry, a "stain-blocking primer" especially
    developed for preventing this type of stain should be applied
    (two primer coats are recommended for severe cases of bleeding
    prior to the finish coat).  Each primer coat should be allowed
    to dry at least 48 hours.



-    Cause of Condition:

    Crazing--fine, jagged interconnected breaks in the top layer
    of paint--results when paint that is several layers thick
    becomes excessively hard and brittle with age and is
    consequently no longer able to expand and contract with the
    wood in response to changes in temperature and humidity.  As
    the wood swells, the bond between paint layers is broken and
    hairline cracks appear.  Although somewhat more difficult to
    detect as opposed to other more obvious paint problems, it is
    well worth the time to scrutinize all surfaces for crazing.
    If not corrected, exterior moisture will enter the crazed
    surface, resulting in further swelling of the wood and,
    eventually, deep cracking and alligatoring, a Class III
    condition which requires total paint removal.

-    Recommended Treatment:

    Crazing can be treated by hand or mechanically sanding the
    surface, then repainting.  Although the hairline cracks may
    tend to show through the new paint, the surface will be
    protected against exterior moisture penetration.


-    Cause of Condition:

    Intercoat peeling can be the result of improper surface
    preparation prior to the last repainting.  This most often
    occurs in protected areas such as eaves and covered porches
    because these surfaces do not receive a regular rinsing from
    rainfall, and salts from air-borne pollutants thus accumulate
    on the surface.  If not cleaned off, the new paint coat will
    not adhere properly and that layer will peel.

    Another common cause of intercoat peeling is incompatibility
    between paint types.  For example, if oil paint is applied
    over latex paint, peeling of the top coat can sometimes result
    since, upon aging, the oil paint becomes harder and less
    elastic than the latex paint.  If latex paint is applied over
    old, chalking oil paint, peeling can also occur because the
    latex paint is unable to penetrate the chalky surface and

-    Recommended Treatment:

    First, where salts or impurities have caused the peeling, the
    affected area should be washed down thoroughly after scraping,
    then wiped dry.  Finally, the surface should be hand or
    mechanically sanded, then repainted.

    Where peeling was the result of using incompatible paints, the
    peeling top coat should be scraped and hand or mechanically
    sanded.  Application of a high quality oil type exterior
    primer will provide a surface over which either an oil or a
    latex topcoat can be successfully used.


-    Cause of Condition:

    Solvent blistering, the result of a less common application
    error, is not caused by moisture, but by the action of ambient
    heat on paint solvent or thinners in the paint film.  If
    solvent-rich paint is applied in direct sunlight, the top
    surface can dry too quickly and, as a result, solvents become
    trapped beneath the dried paint film.  When the solvent
    vaporizes, it forces its way through the paint film, resulting
    in surface blisters.  This problem occurs more often with dark
    colored paints because darker colors absorb more heat than
    lighter ones.  To distinguish between solvent blistering and
    blistering caused by moisture, a blister should be cut open.
    If another layer of paint is visible, then solvent blistering
    is the likely problem.  Whereas, if bare wood is revealed,
    moisture is probably to blame.  Solvent blisters are generally

-    Recommended Treatment:

    Solvent-blistered areas can be scraped, hand or mechanically
    sanded to the next sound layer, then repainted.  In order to
    prevent blistering of painted surfaces, paint should not be
    applied in direct sunlight.


-    Cause of Condition:

    Another error in application that can easily be avoided is
    wrinkling.  This occurs when the top layer of paint dries
    before the layer underneath.  The top layer of paint actually
    moves as the paint underneath (a primer, for example) is
    drying.  Specific causes of wrinkling include: (1) applying
    paint too thick; (2) applying a second coat before the first
    one dries; (3) inadequate brushing out; and (4) painting in
    temperatures higher than recommended by the manufacturer.

-    Recommended Treatment:

    The wrinkled layer can be removed by scraping followed by hand
    or mechanical sanding to provide as even a surface as
    possible, then repainted following manufacturer's application


If surface conditions are such that the majority of paint will have
to be removed prior to repainting, it is suggested that a small
sample of intact paint be left in an inconspicuous area either by
covering the area with a metal plate, or by marking the area and
identifying it in some way.  (When repainting does take place, the
sample should not be painted over).  This will enable future
investigators to have a record of the building's paint history.


-    Cause of Condition:

    Peeling to bare wood is most often caused by excess interior
    or exterior moisture that collects behind the paint film, thus
    impairing adhesion.  Generally beginning as blisters, cracking
    and peeling occur as moisture causes the wood to swell,
    breaking the adhesion of the bottom layer.

-    Recommended Treatment:

    There is no reason for repainting before dealing with the
    moisture problems because new paint will simply fail.
    Therefore, the first step in treating peeling is to locate and
    remove the source or sources of the moisture, not only because
    moisture will jeopardize the protective coating of paint, but
    if left unattended, it can ultimately cause permanent damage
    to the wood.  Excess interior moisture should be removed from
    the building through installation of exhaust fans and vents.
    Exterior moisture should be eliminated by correcting the
    following conditions prior to repainting: faulty flashing;
    leaking gutters; defective roof shingles; cracks and holes in
    siding and trim; deteriorated caulking in joints and seams;
    and shrubbery growing too close to painted wood.  After the
    moisture problems have been resolved, the wood must be
    permitted to dry out thoroughly The damaged paint can then be
    scraped off with a putty knife, hand or mechanically sanded,
    primed, and repainted.


-    Cause of Condition:

    Cracking and alligatoring are advanced stages of crazing.
    Once the bond between layers has been broken due to intercoat
    paint failure, exterior moisture is able to penetrate the
    surface cracks, causing the wood to swell and deeper cracking
    to take place.  This process continues until cracking, which
    forms parallel to grain, extends to bare wood.  Ultimately,
    the cracking becomes an overall pattern of horizontal and
    vertical breaks in the paint layers that looks like reptile
    skin, hence, "alligatoring."  In advanced stages of cracking
    and alligatoring, the surfaces will also flake badly.

-    Recommended Treatment:

    If cracking and alligatoring are present only in the top
    layers they can probably be scraped, hand or mechanically
    sanded to the next sound layer, then repainted.  However, if
    cracking and/or alligatoring have progressed to bare wood and
    the paint has begun to flake, it will need to be totally
    removed.  Methods include scraping or paint removal with the
    electric heat plate, electric heat gun, or chemical strippers,
    depending on the particular area involved.  Bare wood should
    be primed within 48 hours, then repainted.


After having presented the "hierarchy" of exterior paint surface
conditions--from a mild condition such as mildewing which simply
requires cleaning prior to repainting to serious conditions such as
peeling and alligatoring which require total paint removal--one
important thought bears repeating: if a paint problem has been
identified that warrants either limited or total paint removal, the
gentlest method possible for the particular wooden element of the
historic building should be selected from the many available

The treatments recommended (based upon field testing as well as on
site monitoring of Department of Interior grant-in-aid and
certification of rehabilitation projects) are those which take
three over-riding issues into consideration (1) the continued
protection and preservation of the historic exterior woodwork; (2)
the retention of the sequence of historic paint layers; and (3) the
health and safety of those individuals performing the paint
removal.  By applying these criteria, it is evident that no paint
removal method is without its drawbacks and all recommendations are
qualified in varying degrees.


After a particular exterior paint surface condition has been
identified, the next step in planning for repainting--if paint
removal is required--is selecting an appropriate method for such

The method or methods selected should be suitable for the specific
paint problem as well as the particular wooden element of the
building.  Methods for paint removal can be divided into three
categories (frequently, however, a combination of the three methods
is used).

Each method is defined below, then discussed further and specific
recommendations made:

    1.   Abrasive--"Abrading" the painted surface by manual and/or
         mechanical means such as scraping and sanding.  Generally
         used for surface preparation and limited paint removal .

    2.   Thermal--Softening and raising the paint layers by
         applying heat followed by scraping and sanding.
         Generally used for total paint removal.

    3.   Chemical--Softening of the paint layers with chemical
         strippers followed by scraping and sanding.  Generally
         used for total paint removal.


If conditions have been identified that require limited paint
removal such as crazing, intercoat peeling, solvent blistering, and
wrinkling, scraping and hand sanding should be the first methods
employed before using mechanical means.  Even in the case of more
serious conditions such as peeling--where the damaged paint is weak
and already sufficiently loosened from the wood surface--scraping
and hand sanding may be all that is needed prior to repainting.

RECOMMENDED ABRASIVE METHODS (MANUAL):  Putty knife, paint scraper,
sandpaper, sanding block, sanding sponge.  Applicable areas of
building: All areas.  For use on: Class I, Class II, and Class III
conditions.  Health/Safety factors: Take precautions against lead
dust, eye damage; dispose of lead paint residue properly.

-    Putty Knife/Paint Scraper: Scraping is usually accomplished
    with either a putty knife or a paint scraper, or both.  Putty
    knives range in width from one to six inches and have a
    beveled edge.  A putty knife is used in a pushing motion going
    under the paint and working from an area of loose paint toward
    the edge where the paint is still firmly adhered.  In effect,
    "beveling" the remaining layers so that as smooth a transition
    as possible is made between damaged and undamaged areas.

    Paint scrapers are commonly available in 1-5/16, 2-1/2 and
    3-1/2 inch widths and have replaceable blades.  In addition,
    profiled scrapers can be made specifically for use on
    moldings.  As opposed to the putty knife, the paint scraper is
    used in a pulling motion and works by raking the damaged areas
    of paint away.

    The obvious goal in using the putty knife or the paint scraper
    is to selectively remove the affected layer or layers of
    paint.  However, both of these tools, particularly the paint
    scraper with its hooked edge, must be used with care to
    properly prepare the surface and to avoid gouging the wood.

-    Sandpaper/Sanding Block/Sanding sponge: After manually
    removing the damaged layer or layers by scraping, the uneven
    surface (due to the almost inevitable removal of varying
    numbers of paint layers in a given area) will need to be
    smoothed or "feathered out" prior to repainting.  As stated
    before, hand sanding, as opposed to harsher mechanical
    sanding, is recommended if the area is relatively limited.  A
    coarse grit, open-coat flint sandpaper--the least expensive
    kind--is useful for this purpose because, as the sandpaper
    clogs with paint it must be discarded and this process
    repeated until all layers adhere uniformly.

    Blocks made of wood or hard rubber and covered with sandpaper
    are useful for handsanding flat surfaces.  Sanding
    sponges--rectangular sponges with an abrasive aggregate on
    their surfaces--are also available for detail work that

residue properly.

Last Reviewed 2012-09-10