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 finishes and materials. Wet cleaning should not be
carried out when there is any danger of frost within the
(The U.S. Weather Bureau can provide
next several weeks.
average freeze dates for autumn and spring for any part of
the country.) Watch for ground saturation caused by
spraying. It can cause rising damp.
Before wet cleaning:
Make sure that damaged masonry and joints have
o
been repaired and that caulking and mortar are
sound.
o
Use techniques requiring the least amount of
water.
o
Use only plastic or nonferrous tools.
Seal door and window openings with 15-millimeter
o
polyethylene taped all around the opening.
o
Heating Tools. Heat guns and heat plates that have a
working temperature of less than 750F may be used with
caution. Propane torches and other types of open flame
should never be used on any historic building. Many
historic building fires have resulted from their use. In
addition, they can damage old brick, wood, and other
building materials, and break window glass. They also
create hazardous fumes, as they vaporize any lead that may
be present in the paint. Whenever heat is used to remove
paint, a fire watch must be kept on the site for at least
3 hours after discontinuing the heat use. Smoldering
fires can take a long time to show up.
o
Chemicals. Paint-stripping chemicals should be used only
when necessary and by professionals. Although chemicals
are the most effective way to completely strip paint, they
present environmental and health hazards. Chemical
stripping is also slow and often messy. If several layers
of different types of paint must be removed, it may be
necessary to use a different chemical on each layer.
Alkaline strippers (sodium or potassium
hydroxide-based) work best on old linseed oil paints. One
disadvantage of alkaline strippers is that they can cause
efflorescence on masonry surfaces. A weak hydrofluoric
acid cleanser is sometimes applied as a neutralizer before
rinsing thoroughly with water.
Organic chemicals such as methylene chloride are better
for modern paints and finishes such as urethane varnishes
or epoxy. Methylene chloride strippers must be applied as
poultices, or leaching packs, to avoid spreading stains.
They require thorough rinsing. Methylene chloride is a
carcinogen and it should be used only out doors or in
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