Civil Engineering Reference
In-Depth Information
stubborn areas, followed by scraping
and medium pressure lance washing.
Felt-tips on smooth surfaces may be
cleaned with solvent based agents,
and on porous surfaces may yield to
bleach. Proprietary paint strippers
especially need to be selected with
care and first used on small trial
surfaces. Many are based on sodium
hydroxide, which is effective on oil
based paints, but which can leave
harmful deposits in the pores of the
treated substrate.
Plastics can be damaged by some
organic solvents, and painted
surfaces by solutions containing
methylene dichloride. Many surfaces
can be damaged by mechanical
abrasion, which should be used as a
last resort.
Care should be taken with pressure
lances by not using too high a
pressure; otherwise delicate or old
surfaces, such as those on historic
buildings, may be damaged. Heating
the water may be useful, depending
on the nature of the offending graffiti,
and even steam cleaning may be
appropriate with some modern paints.
Anti-graffiti coatings are basically
of two types:
●
Figure 9.35
Paint failure on unstable surfaces of brick masonry
Workmanship
For all painting, but particularly on
wood, it is important wherever
possible to use primers, undercoats
and topcoats from the same
manufacturer.
If the surface is at all friable, it will
need stabilising before paint is
applied, otherwise failure will ensue
(Figure 9.35).
Work on site
Storage and handling of materials
Many paints contain solvents of
various kinds including white spirit,
and anti-graffiti solvents may contain
chlorinated hydrocarbons.
Care is needed with all such
substances in their use in order to
comply with COSSH regulations.
long lasting seals or varnishes,
which may be clear or pigmented
depending on whether the original
surface appearance should be kept
or whether it needs to be
obliterated (in either case, two-
pack polyurethanes are probably
best)
Restrictions due to weather
conditions
There are obvious limitations on the
used of waterborne formulations in
wet weather.
Solvent based paints should not be
used when:
●
there is any visible moisture on the
surfaces to be painted
●
moisture content of wood exceeds
18%
●
air temperature falls below 5 °C
●
Inspection
The problems to look for are:
◊
waxes or polysaccharides which
may give temporary protection
since they tend to be partly or
wholly removed with the graffiti
peeling
●
◊
lack of adequate surface preparation
◊
no sealing coats to bridge cracks
◊
incompatibility of redecoration with
existing
◊
friable substrates
With both types they should be
specified only after examination of
the implications of a process whose
effects might well be irreversible.
Further information is
obtainable
(264,265)
.
◊
micro-organisms affecting painted
wood
wind raises dust
