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Historic Preservation - Technical Procedures

Spectitle:

Primers And Paints For Wood

Procedure code:

0630001S

Source:

Hspg Prepared For Nps - Sero

Division:

Wood And Plastics

Section:

Wood Treatment

Last Modified:

02/24/2012

Details:

Primers And Paints For Wood



PRIMERS AND PAINTS FOR WOOD


This standard includes general information on primers and paints to
be used on interior and exterior wood surfaces.

The primary purpose of paint is to protect wood from deterioration.
To do so, paint manufacturers have developed paint systems which
are made to work together to protect the wood substrate.  These
systems include primers and appropriate, compatible top coats which
can vary depending on the substrate and can vary between
manufacturers.  As a result appropriate primers and compatible top
coats, both from the same manufacturer should be used.  

For information on paint removal from wood, surface preparation and
application procedures, see the following:

-    For guidance on paint removal from wood, see 06400-07-R
    "Chemically Removing Paint From Wood Features", 06400-02-S
    "Supplemental Guidelines for Removing Paint from Interior and
    Exterior Wood Surfaces" and 06400-09-R "Removing Paint From
    Wood Features Using Thermal Methods".

-    For guidance on preparing wood surfaces for painting, see
    06300-02-R.

-    For additional information on the history, properties and uses
    of paint, see 09900-01-S.

-    See 09900-07-S for general guidelines on painting interior and
    exterior surfaces.


***PRIMERS FOR WOOD***

CHARACTERISTICS

-    They prevent certain chemical reactions from occurring between
    the wood and the top coats.  In wood, water soluble
    extractives are often a natural constituent of many wood
    species.  Stain blocking primers, either oil- or water-based
    are especially important to use in these situations.

-    They provide a more stable substrate for the new top coats.

-    They provide a uniform coat allowing more even color coverage
    of the top coats.

-    They bind weathered wood fibers, providing a more stable
    substrate.

-    For areas subject to heavy mildew, mildew resistant primers
    are also available, making the surface less susceptible to
    mildew.


TYPES

Oil/Alkyd Primers:  

-    Must be applied to a completely dry surface.

-    Recommended for use when all of the paint has NOT been removed
    from the surface.  Oil based paint is better able to bind with
    old, chalky paint layers thereby providing a more stable base
    for the finish coats.

-    Recommended for use if the existing paint type is unknown (oil
    or latex?), or if a switch to latex top coat is being made.

-    Appropriate for use on wood surfaces from which all paint has
    been removed but which are subject to less movement, i.e. a
    wall surface rather than a fence.

-    Advantages:

    1.   Have excellent stain blocking capabilities.

    2.   Suited for wood in which the paint has NOT been removed.

Acrylic Latex Primers:

-    Found to be successful when used on wood surfaces from which
    all of the paint has been removed and from wood surfaces which
    are "fresh".  

-    They are especially good for surfaces which are considered
    difficult situations i.e. free-standing elements such as
    fences, columns, balustrades, etc. where wood is constantly
    moving.  

-    They can be applied to wood that is slightly damp.  

-    Advantages:

    1.   More flexible than oil/alkyd paints.

-    Limitations:

    1.   As latex paint dries it shrinks more than oil and can
         literally pull off older, more brittle paint layers as it
         dries.  Therefore, oil/alkyd paints are usually
         recommended for use on wood surfaces where all of the
         paint has not been removed.

-    Products:

    1.   Lucite brand of Forest Products Lab:  Found to be the
         most flexible and is the only acrylic latex primer which
         has a good stain blocker.  

    2.   If Lucite is unavailable and the wood being painted
         contains water soluble extractives (cedar and redwood),
         then an oil/alkyd primer is recommended.  

    3.   If in doubt about the stain-blocking capabilities of a
         selected latex primer, test it.


***PAINTS FOR WOOD***

CHARACTERISTICS

-    Made up of three basic ingredients:  

    1.   A binder - oil or water

    2.   Thinner - mineral spirits, turpentine or water

    3.   Pigment - organic or inorganic

-    To these basic ingredients can be added any number of
    additives to produce specialized paints.  

-    The term vehicle, often used in reference to paint, refers to
    the binder plus the thinner.

TYPES

NOTE:  The paint selected must be from the same manufacturer and
made to be used with the primer selected.  It should also be
selected for use in a specific situation where applicable, such as
using porch and deck enamel when painting porch floors.

CAUTION:  PAINTS CONTAINING ZINC ARE TO BE AVOIDED WHEN PAINTING
WOOD, AS ZINC ATTRACTS MOISTURE.

Oil/Alkyd Paints:  

-    Opaque coatings which use natural oils, such as linseed oil,
    or modern alkyds as the binder.  

-    Alkyds are oil modified resins which dry faster and harder
    than ordinary oils.  

-    They offer the best protection from both liquid and vaporous
    water but become brittle with age and eventually are unable to
    move with the substrate and peel, crack, flake, etc.

-    Advantages:

    1.   Durable.

-    Limitations:

    1.   Longer drying time.

    2.   More difficult to clean up than latex paints.

    3.   Can be odorous, volatile and flammable due to the
         presence of organic solvents.

Emulsion or Latex Paints:  

-    Also known as water based paints, these paints have a latex
    binder which has been emulsified or suspended in water.  

-    Acrylic latex resins are particularly durable and favored over
    polyvinyl acetate and polyvinyl chloride latex resins.  

-    They allow more water vapor to pass through than oil based
    paints and they are more flexible, even over time.
    Nevertheless, they will eventually peel, flake, crack, etc.

-    For optimal results when using acrylic latex paints, make sure
    that for at least the first 24 hours after application, a
    temperature of 500F. can be maintained.

-    Advantages:

    1.   General:

         a.   Easy to clean-up.

         b.   May be thinned with water.

         c.   More flexible than oil/alkyd paints.

         d.   Provide better resistance to mildew because there
              is no oil in the paint.  The oil of oil/alkyd
              paints acts as food for mildew.

              NOTE:  THERE HAVE BEEN CASES WHERE MILDEW HAS
              PROLIFERATED EVEN ON A LATEX PAINT SURFACE;
              APPARENTLY THE TINT USED TO COLOR THE PAINT
              PROVIDED THE NECESSARY FOOD SOURCE.  

         e.   Decreased odor, toxicity and flammability (due to
              lack of organic solvents and thinners).

    2.   Acrylic-based paints:  

         a.   Excellent color and gloss retention.

         b.   Good flexibility and durability.

    3.   Polyvinyl acetate emulsion paints:

         a.   Low cost.

         b.   Excellent color retention.

    4.   Styrene-butadiene paints:  None identified.

    5.   Linseed-oil resin-emulsion systems:

         a.   Easy to make.

         b.   Low material cost.

         c.   Improves paint durability.

-    Limitations:

    1.   General:

         a.   Some emulsion paints require the use of a special
              primer or sealer to seal chalky surfaces and
              prevent peeling of the new coating.

    2.   Acrylic-based paints:  

         a.   Sometimes have poor color retention in dark tints.

         b.   Sometimes combined with alkyd-resins for better
              adhesion; This increases the potential of mildew
              growth.

    3.   Polyvinyl acetate emulsion paints:

         a.   Moderate durability when used alone; Durability is
              increased when the vinyl acetate emulsion is
              blended with other emulsions (i.e. acrylic, linseed
              oil, alkyd-resin).

    4.   Styrene-butadiene paints:

         a.   Normally not used on exterior.

         b.   Tend to yellow with age.

         c.   Not very flexible - grain cracking is a frequent
              problem when applied to wood.

         d.   Not readily available today.

    5.   Linseed-oil resin-emulsion systems:  None identified.

-    Products/Suppliers:

    1.   Benjamin Moore and Co.

    2.   Glidden

    3.   PPG Industries, Pittsburgh Paints

    4.   Pratt and Lambert

    5.   The Sherwin-Williams Company

    NOTE:  There are some paints on the market known as self-
    cleaning paints.  As rainwater runs down the wall the paint is
    slowly worn away.  This is called chalking.  Such paints
    should NOT be used if there is an unpainted surface or
    contrasting color below the painted surface, such as a red
    brick foundation wall.  The streaking can be unsightly.

                         END OF SECTION