Applied external finishes - Walls, Windows and Doors - page 244

recommended because of the risk of
moisture build-up from internal
sources. Further information is
available (263) .

Case study
Deteriorating clear varnish to wood
windows and doors
The light coloured clear finish on some
timber windows used in a house conversion
was badly weathered and eroded by flaking
in places on the exterior of the south
elevation. It was particularly bad on the
dormer sills and framing, where there was
complete loss of film and blue-stain fungal
attack on the timber surface. The failure,
seen in what must be only around two years,
is typical of varnish (Figure 9.33). The use of
varnish for the exterior of hardwood joinery
will give a short lifetime before recoating
becomes necessary on exposed surfaces
and maintenance will be more difficult (see
BRE Digest 286 (195) ). A modern stain finish,
as detailed in BRE Information Paper
IP 5/91 (262) , would have given a longer life to
first maintenance and should be used for
redecoration after first getting rid of the
varnish.

Maintenance
Nothing is for ever, and paint systems
are no exception. Preparation of the
surface should be the watchword.
Breakdowns of paint examined by
BRE investigators on site have even
shown coatings of dirt between the
original and the repainting coats! The
surface had not even had a wash
down, let alone sanding and filling.
Solvent borne paints will require
maintenance every 4-6 years, and
waterborne paints slightly longer at
5-8 years.
Cement based paints will need to
be thoroughly cleaned of algae and
redecorated every 5-7 years,
depending on conditions and a
satisfactory substrate. Chlorinated
rubbers and exterior emulsions
should last about the same, whereas
sprayed textured coatings should last
10 years or even more. For more
information refer to BRE Digest 197.

Case study
Failure of a cement based paint on
brick masonry
The BRE Advisory Service was invited to
inspect the masonry paint failure on some
cottages. Because of severe erosion of the
surface of some of the most exposed
original brickwork, the worst affected bricks
had been cut out and replaced with
commons, which were later painted over
(Figure 0.40).
Failure was also occurring in the less
exposed areas, and considerable erosion
had also occurred in the calcium silicate
bricks (Figure 9.34).

Graffiti removal
It may seem somewhat anomalous to
place a section on graffiti removal in
the external finishes chapter.
However, areas of external walls
subject to graffiti which are difficult
to clean may need to be protected
with a coating which either inhibits
attack or facilitates cleaning.
Hand washing with water and or
detergent should be first tried. There
may be acceptable results when the
offending medium is a water based
one, though this will be rare since the
aim of the 'artist' is usually
permanence.
Media which are most difficult to
remove include aerosol paints and
black waterproof felt-tips, more
especially when they have been
applied to absorbent surfaces.
Aerosols may be removable from
non-porous surfaces with a solvent
cleaner and washing, but solvents
should not be used on permeable
finishes, since the offending graffiti
may be driven further into the
surface. Solvent based gels or pastes
applied as a poultice may help with

Figure 9.33
Deteriorating clear varnish finish on a
hardwood door

Difficulties in painting common
fletton bricks have been known for a
number of years. Failure usually
occurs over the kiss marks, which
have a coarser pore structure than the
remainder of the unit, to which the
first coat of paint bonds imperfectly.
Failure typically has ensued after less
than two years, spreading
progressively over the remainder of
the wall. Most types of paint suffer in
the same way. Painted sand faced or
rustics are less liable to failure.
As much as possible of the paint
should be removed from failed
surfaces before remedial action, such
as rendering, is attempted. Bonding
agents are not recommended, but
expanded metal lathing may offer a
more secure solution. Attempts to
waterproof the wall are not

Figure 9.34
Considerable erosion of painted calcium
silicate bricks

Failure of paint on common fletton bricks
is a well known phenomenon. On the calcium
silicate bricks, the failure had occurred
because of a relatively weak brick affected
by:
●

frost action on wet brickwork, which was
the more likely explanation

salt crystallisation, which was less likely

●

● drying shrinkage of the masonry paint,

which was least likely

Remedial measures were difficult. Scraping
off affected areas and repainting might
work, but it might not. The only way to
ensure a satisfactory long term solution
would be to fix EML to the walls and render
before recoating with paint.