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Timber is a dimensionally
unstable material which expands and contracts with changing moisture
content. The timber surface is eroded by ultra violet light, normally
changing to a grey colour leaving cellulose fibres exposed on the surface.
Timber also provides a source of nutrient for mould growth. A protective
system for timber needs to combat these three sources of aggression;
viz. water, ultra violet light and mould.
Some timbers contain resins and oils which can affect the performance
of paint - i.e. resin pockets in pine which can soften and bleed through
paints; oils in teak which can prevent penetration and adhesion of paint;
anti-oxidants in totara and matai which will inhibit the drying of oil-based
paints.
Reconstituted timber products are subject, to a greater or lesser
degree, to the same degradation as timber.
The majority of these products contain wax to improve water-resistance
and wet-strength. These waxes can be dissolved in hydrocarbon solvents
present in oil-based paints and then be 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.
Sharp edges on timber are very difficult to apply paint to as paint
tends to flow away from edges, leaving weak spots. For best results
these sharp edges should be sanded to a rounded profile.
Naturally occuring anti-oxidants in totara and matai
timber inhibit the curing of oxidatively drying paints including alkyd
enamels, primers, undercoats, oil-based stains. These materials can
transfer through any solventborne system to affect subsequent coats.
Highly filled undercoats can give an impression of having dried without
really curing.
Freshly prepared cedar has unique grain patterns, colours and
colour variations. This changes with time at a rate determined by exposure
condition. Under mild indoor conditions, changes are slow and colour
change slight. Under outdoor exterior exposures the changes can be rapid
and drastic.
Cedar erodes appreciably faster than other timbers mainly due to its
low density, lignin content and extractive content which can also cause
staining. When exposed to normal exterior conditions, solubilising of
the lignin takes place due to the effects of light and water. Bacteria
and enzymes also cause degradation resulting in decomposition and favourable
conditions for the growth of fungus and rot.
Surface preparation:
- Remove all moss and mould:
Thoroughly clean down to remove all loosely adhered material. Treat
areas of moss or mould infestation with Resene
Moss & Mould Killer, correctly diluted with clean water. Leave
for up to forty eight hours to achieve full kill.
For heavy infestations further applications may be needed. Wash thoroughly
with clean water to remove all residue.
- Sand timber surfaces:
Always sand along the grain to remove minor imperfections and any
loose surface fibres. Loose fibres can be assumed to be present if
the timber is left exposed to the weather for more than one week.
Remove dust
.
Sharp edges on timber are very difficult to apply paint to, as paint
tends to flow away from edges, leaving weak spots. For best results
these sharp edges must be sanded to a rounded profile
.
NB: All sanding dusts can be harmful and appropriate protection
should be worn. In particular dust from old lead or chromate based
paint can be injurious to health if inhaled or ingested. Seek expert
advice if the presence of these materials is suspected.
- Optional:
Resene
TimberLock is a multi functional timber treatment that improves
the dimensional stability of the substrate, as well as improving its
fungal, water and ultra violet light resistance. Its use improves
the durability of subsequently applied top coats. It is especially
recommended on cedar that is to be painted, as well as on old weathered
timber.
Apply to bare areas, one coat of Resene
TimberLock Preserver/Conditioner at the spreading rate of 5-10
square metres per litre depending on the timber porosity. Allow twenty
four hours to dry. Remove excess Resene
TimberLock by wiping with a turps dampened rag.
- End grains:
Always coat end grains (i.e. underneath the doors) as this prevents
moisture penetration into the timber surface which will eventually
crack the paint film.
- Moisture:
Generally timber should not be coated when its moisture content is
above 17%. Contact Resene if this is suspected.
- Weatherboard profile:
- Type C is by far the most desirable profile for painting,
and on most weatherboard houses the bottom angle is bevelled.
- Type A is extremely difficult to coat with any paint
system. The profile has two sharp edges and scalloped face becomes
almost horizontal at the bottom of the scallop. This means that on
northern facing walls parts of this scalloped face will always be
at right angles to the sun and therefore subject to maximum ultra
violet light.
The sharp edges however, are the major concern with this weatherboard.
This is because, as with all liquids, the effects of surface tension
will pull the liquid away from the sharp edge. In paint systems this
results in a paint coating on these edges of only 50% of the film
build achieved elsewhere.
- Type B weatherboard does not have such a sharp profile as
type A but it is still difficult to adequately coat the edges, and
the top profile is angled to pick up considerably more ultra violet
light than the vertical surface of the board.
- Type D weatherboard can run vertically, diagonally or horizontally.
It does not have the sharp profile problems of type A and B boards,
but if not properly sealed before fixing in place there is easy access
for water to get behind the board because of the square channel pattern
between them. Any timber movement will expose unprimed timber areas
that can soak up water.
- Where boards are fixed other than horizontal, Resene recommends
that the full system be applied to all sides.
First coat:
If the surface you propose
to paint is already painted and in good condition, then you can go straight
to the Topcoats below.
In most cases Resene
Quick Dry is the recommended primer for maximum flexibility and
durability. Resene Quick Dry must be used as the first coat on
Totara and Matai to seal the naturally occuring anti-oxidants.
Ensure all nail holes, damaged or split timbers are filled with a suitable
filler and applied in accordance with manufacturers instructions. Spot
prime filler when dry with Resene Quick Dry Acrylic Primer Undercoat.
NB: If staining is evident after the application of Resene Quick
Dry Acrylic Primer Undercoat, apply a further coat of Resene
Enamel Undercoat.
Topcoats:
Acrylics are most
suitable for exterior applications being more durable and flexible than
alkyd based paints.
Apply either Resene
Hi-Glo (gloss), Resene
Sonyx 101 (semi-gloss) or Resene
Lumbersider (satin) at approximately 12 square metres per litre.
Select the finish as outlined above depending on the desired gloss level
finish. Click here for further information
on gloss levels.
Follow label instructions at all times.
Stained finish:
Film forming stains
will eventually fail by flaking, which Resene considers unacceptable.
Consequently we recommend non film forming, penetrating stains (such as
Resene
Waterborne Woodsman), even so, excess material must be wiped from
the surface. Semi transparent stains are not as durable as fully pigmented
systems and Woodsman will require maintenance after two summers.
Apply two coats of Resene Waterborne Woodsman, wipe away any excess material
sitting on the timber and allow to dry. For dressed timber a third coat
will be required after three months.
Follow label instructions at all times.
Cleaning up:
Cleaning up waterborne paints is different from solventborne paints
- click
here for detailed instructions on clean up.
More Information:
Find out more information on repainting
weatherboards.
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