Architects memo no.
17: April 1982
specifying for interior painting
As is normal at this time of year when dependability of fine
weather becomes less, more concentration is placed on interior painting.
We have therefore decided to use this memo No. 17 to put together a
collation of ideas and do's and don'ts for this important area.
We feel that the best way to handle this subject is via the various
surfaces used starting with timber.
Interior timber work comes under closer scrutiny than its exterior
counterpart and although its protection requirement is less because
it is out of the weather, very high demands are placed on the surface
appearance. High film build is desirable in order to produce the smooth
finishes desired as is the use of plenty of sandpaper. Interior primer/undercoats
have proved valuable in this field as, not requiring the flexibility
of their exterior counterparts, they can be very highly filled to achieve
high film build.
Filling of timber should be done after the first coat has been applied
and sanding is preferably left until the penultimate coat. High class
interior work often demands four coat finishing so that there is sufficient
body of coating present to withstand fine sanding to a flaw-free surface.
Enamels are preferred and semi-gloss enamels can be used to minimise
any defects left by imperfect sanding.
Use of interior rubbing stains followed by low gloss varnishes is
a much more forgiving system. The Polychromatic effect of stained timber
tends to divert the eye from other minor flaws. The use of Resene Colorwood
stain is fully covered in Architects Memo No. 9.
Fibrous plasters present their own special problems. A thin skin of
chemically modified gypsum plaster is cast on a smooth table precoated
with a mutton fat release agent. This thin skin is then backed up with
sisal re-inforced standard plaster. When taken from the mould some of
the thin skin comes away with the sheet but some is sometimes left behind.
The surface of the sheet could therefore vary from very glossy areas
to dull relatively weak areas. To further complicate things the glossy
areas may have some residual areas of the mutton fat releasing agent.
Any stopping used could also be of quite a different nature to the surface
of the sheet.
Our recommendations are always to lightly sand highly glossy areas
in order to give about 70% dulling. The whole surface should then be
coated with a penetrating sealer such as Resene
Sureseal. Once the Resene Sureseal has been applied the desired
finishing coats, whether waterborne or solventborne, can follow. It
is important with fibrous plastered sealings to ensure that the wadding
has fully dried before coating, otherwise stains may still occur even
through the coat of sealer.
Stopped paperfaced plasterboard presents a similarly non-uniform surface,
which can again feature gypsum plasters. It is important to realise
that gypsum plasters do not have anything like the cohesive strength
of paint films and peeling of waterborne paints directly applied to
gypsum plasters, is not uncommon. The peeling in fact is not loss of
adhesion of paint but loss of cohesion within the surface layer of the
The penetrating sealer recommended over gypsum plaster penetrates
within the crystal structure of the plaster and reinforces it much the
same way as polyester reinforces glass fibres. Although the paper coating
on the paperfaced plasterboard is satisfactory to take paint directly,
it will have a different porosity to the stopping; consequently sealing
of the whole surface is recommended. The paper used on paperfaced plasterboard,
after a period of aging uncoated, will develop a yellowish brown colour
that is in fact water soluble and can migrate through latex paints.
The sealer used also provides a barrier to this dye migration.
Particle boards and fibreboards have, as an integral part of their
manufacture, wax added to the chips or fibre. This is an important part
of the board which greatly adds to the water resistance of the material.
It does however, present a problem when decoration of the board is attempted.
Solvents containing paints or varnishes penetrate some way into the
board and dissolve wax near the surface. As the solvent evaporates from
the surface of the film the wax is deposited on that surface leaving
a fine layer. On egg shell and flat finishes this is scarcely visually
discernable but, as a side effect, the wax layer greatly slows down
the ingress of oxygen to the film. All oil modified film binders dry
by an auto-oxidative process; if access of oxygen is reduced; the drying
period is greatly extended.
Subsequent coatings redissolve this layer of wax and redeposit it
on the new film surface therefore perpetuating the problem. In the worst
instances, when a gloss enamel finish is required, a dull patchy surface
can result that could have drying periods extended up to many days.
It is imperative when coating such boards to use a sealer that will
seal off the wax content and not allow the problems outlined above to
occur. The obvious choice is waterbased materials. Resene has two products
on their range that they recommend, Resene
Particle Board Sealer and Resene
Quick Dry Acrylic Undercoat.
The use of waterborne paints on particle boards can result in some
initial chip raising that may necessitate sanding and the use of an
extra coat of sealer. An annoying problem occurs when veneered custom
wood is used for profiled doors with the unveneered edge of the custom
wood routed to the desired profile. The veneer could be satisfactorily
painted but the exposed custom wood must be treated with a suitable
sealer in order to avoid this area being affected by wax.
One of the newer boards to come onto the market is called Sabreline.
Work to date on this material seems to indicate that wax is not a significant
problem with it but it is an extremely porous surface to paint. In our
opinion an extra coat of undercoat or primer/undercoat is required to
bring it up to an acceptable painted surface.
Obviously with a subject such as we have attempted to tackle, not
all surfaces met will be covered in this memo. Should you have any specific
query, please contact our technical staff for additional advice.
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