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SERV I CES & TREATMENT :
PROTEC T I ON & REMED I AL TREATMENT
4.1
development of a wave of ‘compliant
coatings’ including water-borne dispersions,
hybrid products and high-solids coatings.
The growing availability of such products
has gradually changed the way that
modern consumers perceive the role and
performance of protective coatings for
joinery. This has been especially noticeable
in the case of the so-called ‘micro-
porous’ or ‘breather’ paints, which have
proliferated in response to the development
of water-based compliant coatings.
The notion of micro-porosity stemmed
from the need to control and reduce the
build-up of moisture in joinery to below 21
per cent, which is generally accepted as the
point at which timber becomes susceptible to
attack from wood-destroying organisms.
Before the advent of micro-porous
coatings, alkyd paints were the predominant
type of coating for the protection of window
joinery. Alkyd resins are polyesters which
have been chemically reacted with a drying
oil. This process results in the formation of a
three-dimensional interconnecting network
of molecular chains which resists the passage
of water and forms a hard film. It has the
benefit of being easy to clean with solvents
and is less prone to sticking or blocking when
freshly painted. However, its main drawback
is that it becomes brittle with age, becoming
progressively less responsive to dimensional
movements in the woodwork. When this
occurs, cracking of the paint film around
joints is the usual result, allowing rainwater
to be drawn into the wood by capillary
attraction at the point of fracture.
Once inside, the water has no effective
means of being vented out since it cannot re-
emerge past the alkyd paint. The result is that
moisture builds up within the fabric of the
joinery to a point where it can both trigger
the development of rot and interfere with
the surface adhesion of the intact coating,
causing its eventual delamination in the form
of peeling and flaking – a scenario which is
all too familiar in the context of restoration
and repair work.
The production of alkyd paints has
suffered a decline with the rise of water-
borne compliant paints in much the same
way as the invention of paints based on
synthetic resins signalled a similar decline
in the use of linseed oil paints. Against the
backdrop of current VOC legislation this may
seem surprising, given that the production
of linseed oil paints offers a VOC-compliant
alternative with less embodied energy than
paints based on synthetic resins.
LINSEED OIL PAINTS
Linseed oil is a carrier which was commonly
used in paint formulations until the mid-
20th century, and is still often used in
alkyd systems to make the paint more
fluid, transparent and glossy. It is available
in varieties such as cold-pressed, alkali-
refined, sun-bleached, sun-thickened, and
polymerised (also known as stand oil).
The use of linseed oil paint can offer
significant advantages over synthetic resin
systems, not least in terms of longevity.
According to some accounts it can last 15
years or more without maintenance. This
is attributable to the fact that it is more
‘extensible’ (more elastic, less prone to
brittleness) and can better accommodate
shrinkage or swelling movements in wood
before requiring maintenance.
However, such performance benefits
seldom come without a price. In the case of
linseed oil paints, they are more difficult to
apply on account of their lower viscosity.
Application is also labour-intensive due to
the attention required to avoid runs and a
propensity of the finish to incorporate dust
and surface imperfections (nibs) because it
tends to ‘creep’ over them.
Linseed oil paints must also be applied
very thinly to avoid wrinkling on drying and
have a tendency to skin in the can. Once
cured they are softer and may have inferior
resistance to surface abrasion, although this
can be offset by fortifying the formulation
with resin additives such as pine rosin, amber
or semi-fossilised Kauri pine resin.
Arguably the greatest disadvantage
of linseed oil paints is their tendency to
disfigure on account of the growth of surface
moulds and yeasts. This is probably caused
by the paint allowing moisture into the
substrate, enabling natural sugars, present
in the wood, to migrate to the surface where
they can be assimilated by the micro-
organisms, but it may also be caused by the
build-up of dirt deposits which can provide
another source of nutrients and minerals.
When used as a wood finish on its
own, linseed oil dries slowly and shrinks
little upon hardening. It does not deposit
a discrete film over the surface as varnish
does but is absorbed into the surface of
the wood, leaving a shiny but not glossy
surface that enhances the visual contrast
of the grain of the wood. A linseed oil
finish is easily repaired but offers little or
no protection against scratching. Linseed
oil finishes are less effective than paints
based on synthetic resins at preventing
the uptake of moisture, in either liquid
or vapour form, into the joinery.
BURNT SAND MASTIC AND
LINSEED OIL PUTTY
The ingress of moisture into exterior joinery
not only occurs through open joints or via
the external coating but can also occur at
masonry junctions. Traditionally these routes
of potential ingress were sealed using linseed
oil putty made from oil and whiting (chalk)
or burnt sand mastic made from roasted sand
mixed with lead-based ‘driers’ (now replaced
with synthetic driers) and chalk.
The advantage of burnt sand mastic
over linseed oil putty as a peripheral sealant
around window and door frames is that
linseed oil putty oxidises and becomes
brittle with age and exposure to air, a process
which can be slowed, though not prevented,
through the application of a paint coating
over the putty.
Burnt sand mastic can be slow to set
if made without driers and forms a tough,
resilient outer layer as the mastic sets with
a more elastic core making it a far more
extensible material than cement mortar.
It bonds well to brick, stone and wood and
therefore makes an excellent sealant for
wood joinery set into masonry buildings. Its
appearance complements stone and masonry
facades more sympathetically than linseed
oil putty. The sealant is generally used as a
facing over a bedding of hair lime plaster or
compressible filler such as ‘oakum’, making a
wind-proof and water-tight peripheral seal.
Today, burnt sand mastic has largely
been superseded for new-build contracts by
synthetic sealants such as silicone, acrylics
and butyl mastics, each with their own
specific properties. The silicones remain
permanently elastic and when applied
correctly offer a more effective means of
sealing frame joints than acrylic sealants,
which are shorter-lived, or butyl mastics,
which do not set hard.
While the silicones offer a more
effective alternative to burnt sand mastic
in terms of their intrinsic ability to
exclude moisture, they are not, as a rule,
overpainted and may, therefore, not offer
an aesthetically sympathetic sealing
solution which is in keeping with the
visual character of historic masonry.
PETER KACZMAR
BSc MSc is responsible
for scientific research on wood coatings
and timber treatments at the Timber
Research and Development Association
(see
BM TRADA, page 45)
and has co-ordinated
a number of UK and European research
projects in wood-coatings technology. His
sphere of expertise includes the installation
and maintenance of wood floors and floor
systems. More recently his research remit
has extended to include work on timber
modification and stabilisation.
In many European countries, linseed oil is a
traditional finish for fine joinery where protected
from the weather.