Village Squire, 1980-10, Page 16Sandblasting
Here's why it's not such a good idea
BY SUSAN DUNLOP
Sandblasting gives an old building a quick brightening up.
But is it always a good idea?
Take a walk down a commercial or residential street in any
Ontario town. Amongst the aging old buildings which now wear
a weathered coat. you'll eventually conte across one with a
spanking new, freshly -scrubbed facc. Beautiful!
Or is it? Take a closer look.
These buildings with their new facelifts may quickly be losing
their faces through deterioration caused by the inappropriate
cleaning methods. The corners of masonry can become rounded.
the surfaces develop pock marks and actually collect much more
dirt than was ever there before the cleaning.
POTENTIAL PROBLEMS
Grit blasters, grinders, and sanding discs all operate by
abrading the dirt off the surface of the masonry. rather than
reacting with the dirt and masonry as in water and chemical
methods. Since the abrasives do not differentiate between the
dirt and the masonry, some erosion of the masonry surface is
inevitable. A skilled operator can minimize this erosion. but in
the case of brick, soft stone. detailed carvings. or polished
surfaces, even minimal erosion is highly damaging.
Brick, a fired product, is hardest on the outside where the
temperatures were highest; the Toss of this "skin" ot the ortck
exposes the softer inner portion to more rapid deterioration.
Abrasion of smooth surfaces removes the polished quality of the
stone. Such mechanical methods as sandblasting should never
be used on these surfaces and only with extra caution on others.
Mortar joints also can be eroded by mechanical cleaning. In
some cases the damage is visual, such as Toss of joint detail or
increased joint shadows. Repointing is often necessary after
cleaning because erosion permits increased water penetration.
WATERPROOF AND WATER REPELLENT COATINGS
The most common defense for sandblasting is that a
waterproof coating applied to the surface after cleaning will
prevent erosion. Indeed, a sandblasted wall may become so
porous that a silicone coating is prescribed. What is little known
is that these coatings can cause greater deterioration ot the
masonry than that caused by pollution. so the treatment may be
worse than the problem one's attempting to resolve. Coatings do
not solve the problem of water penetration to the interior. In fact.
coatings will often trap water inside the masonry, and will
eventually cause great damage. Also. in some cases the
coatings actually retain dirt more than uncoated masonry, which
will, of course. accelerate deterioration.
Masonry coatings are of two types: waterproof coatings and
water repellent coatings. Waterproof coatings seal the surface
from liquid water and from water vapour; they are usually
opaque such as bituminous coatings and some paints. They do
not cause problems as long as they exclude all water from the
masonry.
Water trapped inside the wall will freeze in cold weather and
cause serious mechanical disruption. such as spalling. In
addition. the water will eventually get out by the path of least
resistance. If it seeps out to the interior it will cause damage to
interior finishes; if it escapes through cracks in the waterproof
coating. it can lead to damage from the build-up of salt.
Water repellent coatings do not seal the surface to water
vapour. so it can enter the wall as well as leave it. Once inside
the wall. vapour condenses into liquid water at cola spots. Salts
frequently are present in masonry: liquid water can dissolve
these salts and carry them to the surface. The presence of a water
repellent coating. however. prevents the water and dissolved
salts from reaching the exterior. The salts are deposited slightly
behind the surface as water evaporates through the pores. Over
several years. salt crystals will grow and develop substantial
pressures which will scall the masonry.
OTHER CLEANING METHODS
Water and chemical cleanings aren't necessarily the perfect
alternatives to mechanical cleaning. They too can cause serious
problems. Chemicals in the water. such as iron and copper. can
disfigure the surface of a building. Water methods cannot be
used daring periods of cold weather since frozen water within the
masonry will cause spalling and cracking. In shite of these
potential problems. water methods are considered the simplest.
safest and least expensive.
PG. 14 VILLAGE SQUIRE/OCTOBER 1980