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Recognising the importance of correct surface preparation for
any successful paint job, Resene included a variety of detailed
information sheets on this subject in their Manual. From the many enquiries
we receive on this subject it appears that information on this subject
requires further dissemination. Therefore for this and some further
memos we intend to reproduce these articles on surface preparation starting
with Timber.
General
Timber is a dimensionally unstable material that expands and contracts
with changing moisture content. The timber surface is eroded by ultra
violet light, normally changing to grey colour, and leaving cellulose
fibres exposed on the surface. Timber also provides a source of nutrient
for mould growth. A protective system for timber needs to be able to
combat these three sources of aggression, viz. water, ultra violet light,
and mould.
Some timber contain resins and oils that can affect the performance
of paint; such as resin pockets in pine that can soften and bleed through
paints; oils in teak that can prevent penetration and adhesion of paint
anti-oxidants in totara and matai that will inhibit the drying of oil-based
paints.
Reconstituted timber products are subject, to a greater or lesser
degree, to the same degradations as timber. The majority of these produces
contain wax to improve water resistance and wet-strength. These waxes
can be dissolved in hydrocarbon solvents present in oil-based paints,
and are then deposited on the film surface when the solvent evaporates.
This wax layer can upset the gloss and finish of the paint system and
seriously retard the drying of oil-based paints by blocking the ingress
of oxygen needed for curing.
Surface Preparation
If mould is present, treat with Resene
Moss & Mould Killer. sand the surface (always along the grain),
to remove minor imperfections and any loose surface fibres. Radius all
edges.
Loose fibres can be assumed to be present if timber is left uncoated
in the weather for more than one week.
N.B.
- Always coat any end grain prior to erection.
- Never coat timber when its moisture content is higher than 16%.
- Always carry out filling procedures after priming.
Refer D82
in the Resene One-Line Specification manual for detailed instructions.
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