Page 44 - Historic Churches 2013

44 BCD Special Report on Historic Churches 20th annual edition CATHEDRALC O M M U N I C A T I O N S
Principal paint options
PAINT TYPE TYPICAL COMPOSITION CHARACTERISTICS
Top and intermediate coats
Lead-based paint
The traditional oil paint used historically
White lead (lead carbonates and
sulphates) in linseed oil
Other ingredients may include colour pigments
(eg metal oxides) and drying agents
Solvent: turpentine (a volatile distillate of pine resin)
Slow drying
Moderately durable (3–7 years)
Distinct lustre
Low VOCs but highly toxic due to lead content
Linseed oil paint
Traditional oil paint but without the lead
Titanium dioxide or zinc oxide and
sometimes chalk in linseed oil
Other ingredients may include colour pigments
(eg metal oxides) and drying agents
Solvent: none required
Slow drying
Moderately durable (3–7 years)
Low environmental impact
Alkyd oil paint
Developed in the 1920s and one of the most
widely available oil paints in use today
Alkyd resins of varying chemistry (typically
an oil-modified polyester), which cure by
film-formation on exposure to air
Oxides of zinc or titanium and other pigments
and drying agents such as metal napthenates
Solvent: white spirit or hexane
Moderately durable (3–7 years)
Offers better film thickness and drying
speed than traditional oil paints
Moderate to high VOCs
2-pack polyurethane paints
Developed in the 1930s and available from specialist
manufacturers, these paints are usually used
with 2-pack epoxy undercoats and primer
Various polyester polyurethanes or acrylic
urethanes formed by the reaction of an
isocyanate curing agent with a selected base resin
containing reactive hydroxyl (OH) groups
Oxides of zinc or titanium and other pigments
Proprietary solvents based on naphtha or
specific solvents such as butyl acetate
Highly durable – 25 years +
Offer excellent film-thickness and fast drying
Wider colour range and more UV stable
than 2-pack epoxy coatings
Moderate to high VOCs
2-pack epoxy paints
Another 1930s discovery with great durability,
but prone to discolouration and chalking
in UV light, so best for undercoats
Epoxy resins of varying chemical formulations
according to requirements, which harden by
reaction with an isocyanate curing agent
Oxides of zinc or titanium and other pigments
Various highly volatile organic solvents such as xylene
As 2-pack polyurethane above but unsuitable
for external use as a topcoat due to chalking
Low (<250g/l) VOC options available
Primers
Traditional lead-based primer Red lead in linseed oil White and red lead acts as a corrosion inhibitor
Modern primers
Only primers compatible with the top coat should
be used, as specified by the manufacturer
2-pack epoxy pigmented with inhibitive anti-
corrosive materials such as zinc phosphate
Low (<250g/l) VOC options available
Aluminium paint - metallic aluminium
flake in an alkyd or epoxy resin base
Metal flake enhances barrier protection
Zinc-rich primers containing high levels of
metallic zinc in an alkyd or epoxy resin base
Metallic zinc provides sacrificial corrosion protection
can be engineered to do so. Leaves and scrolls
in wrought ironwork make it particularly
vulnerable to water-trapping. Careful use
of lead putties to fill pockets will reduce the
problem. Epoxy fillers are used to seal joints.
Spray applications produce the best results
in the workshop. Smaller one-off items tend
to be brush-painted. Two-pack paints must
be mixed in the correct ratios and used before
they ‘go off’. It is this chemical reaction that
produces the fast-curing characteristic, enabling
work to be handled soon after painting.
For paint to perform as an effective barrier
it must flow over the work to form a continuous
wet film of the correct thickness, and it will
then dry as a continuous protective coating.
A poor spraying technique will produce patchy
results. If a paint film is too thick it will tend
to sag and run. A spray gun held too far from
the work or used in short blasts will cause
the paint to hit the surface ‘dry’ producing
a rough surface that has no film continuity.
Skilled brushwork will produce the continuous
film required without patchiness, runs or
drags. Small rollers may be used on palings.
Paint ‘misses’ are avoided by using
different shades for each coat (for example,
an off-white undercoat for a white topcoat).
Specifications sometimes set out the shades
to be used for each coat so that they can be
readily identified for inspection and approval.
Film thicknesses can be measured
with equipment ranging from simple
pocket mechanical devices through to
sophisticated electronic instruments such
as those manufactured by Elcometer.
During on-site painting it is important
to ensure that adjoining surfaces are properly
protected. Where ironwork meets masonry,
the latter should be masked off. Method
statements should state how general fabric
will be protected (not simply that it will
be) and how spills would be addressed.
Aftercare
Finally, there are a few simple measures
to keep new paintwork in good order:
• Provide for repair or replacement of locks
in specifications for gates, including the
number of keys required. This eliminates
the need for chains and padlocks which
can quickly damage paintwork and look
unsightly.
• Consider how gates are to be held open.
Various types of holdback latch can be made
and installed with the refurbished gates.
• Gates may need adjusting at the hinges
from time to time so they close correctly.
Address this promptly should the need
arise.
• Inspect all ironwork annually and touch-
in any paint chips and scratches with
an appropriate product. Clear away any
vegetation (ivy growth, leaf debris) that
might prevent ironwork drying out after rain.
John Wilson is contracts director at Calibre
Metalwork Limited. His interest in conservation
began while he was working for a commercial
developer converting Wapping and Waterloo
dock warehouses in Liverpool for residential
use. His interest in the building preservation
trust movement includes raising funds to rescue
the Grade II* Plaza Cinema in Stockport.
It is important to achieve an even, continuous coating
with the brush.