Tools and Techniques
A heavy-duty heat gun was approved for the contractor’s use (see figure
3). The metal encased unit featured variable heat control (500-750°F) through
adjustable side vents, and a cool down switch, which extends the life of the
replaceable heating elements. Since these elements can have a working life as
short as 60 to 80 hours, extra elements were purchased and were available at
the site for immediate replacement of those in the five heat guns used in the
project.
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| Figure 3. Heat guns, putty knives with corners rounded, and paint scrapers
were used in paint removal. Photo: Michael Lee |
Each unit operated on its own 20-ampere circuit to prevent constant tripping
of breaker switches due to the high-energy consumption of the heat guns (14 amps,
120 volts). Temporary weatherproof electrical outlets with ground fault interrupters
were installed in the most advantageous locations to prevent voltage drop due
to long extension cords.
Workers used alternative paint removal methods on some portions of the structure,
including porch ceilings and cornices, because of the increased risk of overheating
cavities in these locations. They exercised particular caution around windows
and doors where siding butt-joined the trim. An additional concern was the potential
for dust and other debris to overheat in the hollow areas behind the casing,
such as in the sash weight boxes. For safety, siding paint was removed thermally
no closer than 6 inches from window and door trim and alternate paint removal
or feathering methods were employed for the remainder. The workers used chemical
strippers around glass instead of heat guns to avoid heat stress fractures.
Special care was taken in the removal of paint from decorative millwork. Scorching
of decorative wood trim can occur easily, especially if the heat gun is trained
on the same area too long in an attempt to remove all the irregular paint build-up.
The heat gun was set at the lowest temperature and adjusted higher as necessary
to compensate for the thickness and condition of the paint.
During paint removal, the scraped paint tended to adhere to the putty knife,
slowing the paint removal process. The workers tried several putty knives and
scrapers, but none was successful in shedding the paint residue from the blade.
Therefore, the knives had to be periodically cleaned of gummed paint with another
putty knife. For siding, a 2 to 2 ½ inch-wide rigid putty knife worked
best. Narrower blades were used for millwork and tight spots. To reduce chances
of the workers gouging the wood with the putty knives, the edges of the blades
were rounded using a grinder.
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| Figure 4. Method used to join two heat guns for
simultaneous use. Drawing: Alan O’Bright |
The workers used two techniques for thermal paint removal with equal success.
In the first case, one worker alternately heated and scraped the surface. Time
was lost in this method because the heat gun had to be set down periodically
in order for the worker to clean his putty knife. The second technique, developed
by the contractor’s site supervisor, was to fasten two heat guns together
side by side using metal bars bolted to each gun base (see figure 4). In this
way one worker heated the surface while another scraped in a continuous process.
This method worked very well for expansive areas of siding and attained an average
rate of 8 to 10 square feet per crew hour.
On siding in tight areas where one
worker operated a single heat gun, paint was removed at a rate of approximately
4 square feet per crew hour (see figure 5).
The siding was inspected for rot and cracks, and suspect sections were marked
for repair or replacement. The marked sections were passed over in the paint
removal process.
Fire Safety
Thermally removing paint using a heat gun does carry certain fire risks that
require precautions both in the planning as well as in the execution of the
work. With an ignition temperature of approximately 200-250°F, the wood
itself can ignite from the hot air blast, leading to potentially serious fire
damage to the historic building.
In assessing the risk of the wood igniting, a number of factors need to be
taken into account. The moisture level of the wood definitely affects the temperature
at which the wood ignites. Wall studs behind the siding that are adjacent to
high temperature heating pipes would be very dry compared to the siding. If
the heat from the gun did not dissipate fast enough within the wall cavity,
studs or deadening boards could begin to smolder in particularly hot spots
or areas of very dried wood, even though the siding is not immediately affected.
And where there is insulation in the wall, heat build-up would be greater,
thereby increasing the fire risks. Even the daytime temperature and prevailing
breezes need to be considered, since cooler temperatures and a mild breeze
will help cool the siding faster. On the other hand, strong winds will make
it more difficult to remove the paint, increasing the fire risks in a variety
of ways.
Another factor to be considered is the surface condition of the siding. Very
rough edges are more susceptible to ignition than smooth surfaces. A more common
problem that must be taken into account not only in planning but also throughout
the work is the tendency of the laborers to get impatient or careless, directing
the heat gun in one spot too long or adjusting the heat gun to a higher temperature.
In addition to the possibility of igniting the wood, there is the even greater
risk of ignition of flammable debris commonly found in wall cavities and behind
cornices. Debris such as bird and rat nests, builder’s trash, accumulated
dust and building material waste can all be more flammable than the wood siding.
Examining selective areas of the wall cavity and cornices prior to selecting
a paint removal method can establish the extent of potential fire risk from
debris and building material.
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| Figure 5: Using scaffolding, workers remove the
paint with heat guns. Cracked or deteriorated siding was repaired or
replaced.
Photo: Alan O’Bright |
Additional precautions need to be taken in the course of work. Both the work
crew and park staff at the Truman home were thoroughly familiarized with
the fire risks involved. Besides using scrapers or chemical strippers in
the areas
of highest risk, workers were instructed to avoid overheating the wood. This
tends to occur at uneven wood surfaces, such as found in decorative trim
or in cornices. Since workers tend to get overly confident and very casual
as
the job proceeds, someone on the crew should be assigned responsibility as
the “fire-safety inspector.”
Suitable fire-fighting equipment should be readily available. At the Truman
home, carbon dioxide and water fire extinguishers were within immediate reach
of every workstation where a heat gun was being used. The contractor added
glycol to the water extinguishers during cold weather work to prevent freezing.
In addition, a long garden hose was kept near the work site during warm weather.
Since debris and wood will tend to smolder for a number of hours before breaking
out into flames, the building should be equipped, if possible, with a temporary
fire detection system in the attic eaves and adjacent to exterior walls.
Furthermore, paint removal using heat guns should stop at least several hours
prior to the
site being vacated each evening, to increase chances of early detection of
any smoldering fire. The area of the day’s work must be carefully inspected.
And finally, if there is a night watchman, extra diligence should be demanded
during the weeks when paint removal is occurring. Health and Safety Considerations
Since most of the pre-1950s paint used on the house exteriors is lead-based,
additional special precautions are necessary in removing such paint. When thermally
removing paint, workers should use respirators approved by the National Institute
for Occupational Safety and Health (NIOSH) that have cartridges specially designed
to filter lead. For projects as large as the Truman home, replacement filters
available at the site are recommended to permit the necessary frequent changes
of filters without interrupting the work schedule. Workers should wear separate
clothing for paint stripping and provide for full leg and arm protection. An
industrial vacuum cleaner placed outdoors permits periodic cleaning of clothes
and the work area.
Additional time should be provided for workers to clean up properly before
eating, and no eating should take place within the work area.
As at the Truman home, large plastic drop sheets should be used to collect
the paint chips and prevent the lead-based paint from being deposited in the
soil. Collected each day, paint chips need to be safely stored for disposition
in accordance with the local and state health boards’ guidelines for
toxic waste.
To keep the inside of the building from being overexposed to toxic vapors when
removing exterior paint, it is necessary to close all doors and windows and
to turn off intake fans. Workers should not be permitted to walk into the building
wearing contaminated clothes. They must also wash their clothes separately
from the rest of their laundry. As a final precaution, the work area should
be closed to all but essential personnel, but in any event, caution must be
exercised to prevent small children, pregnant women, and people in ill health
from entering the site.
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