Thermal Methods
Where exterior surface conditions have been identified that warrant
total paint removal such as peeling, cracking, or alligatoring,
two thermal devices--the electric heat plate and the electric
heat gun--have proven to be quite successful for use on different
wooden elements of the historic building. One thermal method--the
blow torch--is not recommended because it can scorch the wood
or even burn the building down!
Recommended Thermal Methods
 A heat plate was used on the cornice to remove paint. Photo: NPS files. |
Electric heat plate: The electric heat plate operates
between 500 and 800 degrees Fahrenheit (not hot enough to vaporize
lead paint), using about 15 amps of power. The plate is held close
to the painted exterior surface until the layers of paint begin
to soften and blister, then moved to an adjacent location on the
wood while the softened paint is scraped off with a putty knife
(it should be noted that the heat plate is most successful when
the paint is very thick!). With practice, the operator can successfully
move the heat plate evenly across a flat surface such as wooden
siding or a window sill or door in a continuous motion, thus lessening
the risk of scorching the wood in an attempt to reheat the edge
of the paint sufficiently for effective removal. Since the electric
heat plate's coil is "red hot," extreme caution should
be taken to avoid igniting clothing or burning the skin. If an
extension cord is used, it should be a heavy-duty cord (with 3-prong
grounded plugs). A heat plate could overload a circuit or, even
worse, cause an electrical fire; therefore, it is recommended
that this implement be used with a single circuit and that a fire
extinguisher always be kept close at hand.
Electric heat gun: The electric heat gun (electric hot-air gun)
looks like a hand-held hairdryer with a heavy-duty metal case. It has an electrical resistance coil that typically
heats between 500 and 750 degrees Fahrenheit and, again, uses
about 15 amps of power which requires a heavy-duty extension cord.
There are some heat guns that operate at higher temperatures but
they should not be purchased for removing old paint because of
the danger of lead paint vapors.
The nozzle on the electric heat gun permits hot air to be aimed into cavities on solid decorative surfaces, such as this carriage house door. After the paint has been sufficiently softened, it can be carefully removed with a scraper. Photo: NPS files.
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The temperature is controlled
by a vent on the side of the heat gun. When the vent is closed,
the heat increases. A fan forces a stream of hot air against the
painted woodwork, causing a blister to form. At that point, the
softened paint can be peeled back with a putty knife. It can be
used to best advantage when a paneled door was originally varnished,
then painted a number of times. In this case, the paint will come
off quite easily, often leaving an almost pristine varnished surface
behind. Like the heat plate, the heat gun works best on a heavy
paint buildup. (It is, however, not very successful on only one
or two layers of paint or on surfaces that have only been varnished.
The varnish simply becomes sticky and the wood scorches.)
Although the heat gun is heavier and more tiring to use than the
heat plate, it is particularly effective for removing paint from
detail work because the nozzle can be directed at curved and intricate
surfaces. Its use is thus more limited than the heat plate, and
most successfully used in conjunction with the heat plate. For
example, it takes about two to three hours to strip a paneled
door with a heat gun, but if used in combination with a heat plate
for the large, flat area, the time can usually be cut in half.
Although a heat gun seldom scorches wood, it can cause fires (like
the blow torch) if aimed at the dusty cavity between the exterior
sheathing and siding and interior lath and plaster. A fire may
smolder for hours before flames break through to the surface.
Therefore, this thermal device is best suited for use on solid
decorative elements, such as molding, balusters, fretwork, or
"gingerbread."
Thermal Methods Not Recommended
Blow Torch: Blow torches, such as hand-held propane or butane torches,
were widely used in the past for paint removal because other thermal
devices were not available. With this technique, the flame is
directed toward the paint until it begins to bubble and loosen
from the surface. Then the paint is scraped off with a putty knife.
Although this is a relatively fast process, at temperatures between
3200 and 3800 degrees Fahrenheit the open flame is not only capable
of burning a careless operator and causing severe damage to eyes
or skin, it can easily scorch or ignite the wood. The other fire
hazard is more insidious. Most frame buildings have an air space
between the exterior sheathing and siding and interior lath and
plaster. This cavity usually has an accumulation of dust which
is also easily ignited by the open flame of a blow torch. Finally,
leadbase paints will vaporize at high temperatures, releasing
toxic fumes that can be unknowingly inhaled. Therefore, because
both the heat plate and the heat gun are generally safer to use--that
is, the risks are much more controllable--the blow torch should
definitely be avoided!
Summary of Thermal Methods
Recommended: Electric heat plate, electric heat gun.
Applicable areas of building: Electric heat plate--flat surfaces
such as siding, eaves, sash, sills, doors. Electric heat gun--solid
decorative molding, balusters, fretwork, or "gingerbread."
For use on: Class III conditions.
Health/Safety factors: Take precautions against eye damage and
fire. Dispose of lead paint residue properly.
Not Recommended: Blow torch.
"Exterior Paint Problems" an Historic Preservation Brief
