Exterior paint and finishes that last in New Zealand

I came to this work as a painter, and surfaces are still my trade. Paint on a New Zealand house works harder than paint at the same latitude in the northern hemisphere, and how long it lasts is decided mostly before the first coat goes on.

What our climate does to paint

Sun. Summer UV in New Zealand runs about 40 per cent higher than in countries at similar latitudes in the northern hemisphere. UV breaks down the binder that holds paint together, which is why the north and west walls go first.

Salt. Under NZS 3604, the standard for timber-framed buildings, land within 500 metres of the sea, harbours included, or within 100 metres of a tidal estuary or sheltered inlet, sits in the highest sea-spray zone. BRANZ publishes the maps. Salt settles as a film that holds moisture against the paint. Rain rinses open walls, but under the eaves, on soffits and in sheltered corners nothing does, and that is where coatings and fixings fail first.

Wet, then dry. Timber takes up water and swells, then dries and shrinks. The paint has to move with it, and it cracks first at joins, end grain and sills.

None of this can be stopped. It can be slowed, and nearly all the slowing is preparation and washing.

What a good job should give you

Resene's guidance is that a quality three-coat acrylic system, a primer and two topcoats, should perform on timber weatherboards for 7 to 10 years, and 5 to 7 on sills and sloping surfaces that take more sun. An oil-based or alkyd system gives 4 to 6. Where a house lands in that range depends on the preparation, the exposure, and whether it is washed every year.

Preparation decides the life

The paint is the cheapest part of the job. What decides how long it lasts is mostly underneath: a great many failures on timber come from the timber moving or breaking down. A good job includes:

• A clean surface. Mould, salt and chalk left under a new coat will lift it. Wash, treat any mould, rinse, and let it dry.
• Sound timber. Soft or split boards replaced, nail heads punched and filled, open joins and end grain sealed. Paint over rotten timber fails with the timber.
• Bare timber primed quickly. Timber left in the weather for more than a week grows loose fibres and needs sanding along the grain before priming. Grey, weathered wood goes back to fresh timber. Freshly sanded timber is primed the same day, before the dew gets under it.
• Old gloss sanded so the new coat has something to grip.
• The full system. The primer the system calls for, then two topcoats at the spread rate on the tin. Thin coats mean a shorter life.
• The right weather. Waterborne paint above 10°C, ideally 15 to 20°C, out of direct sun and extreme heat, never onto damp timber, and finished early enough to dry before the temperature drops at sunset. Resene's test: if the washing on the line isn't drying, neither will the paint.

Ask for the preparation to be written into the quote, item by item. It shows you which quotes are for the same job.

Older homes and lead

If your home was built in the 1980s or earlier, Health New Zealand's advice is to presume it has lead-based paint in its layers, with the most in paint from before 1965. The risk comes when it is disturbed. The guidance is wet sanding rather than dry, no abrasive blasting on anything built or painted before 1970, no blow torch, which releases lead fumes, no burning of paint debris, and children and pets kept away. A test kit from a paint shop tells you what you have. Before any painter starts on an older house, ask how they will contain the dust.

Dark colours on weatherboards

Dark colours are hard on timber. A dark wall in full sun gets hot enough to move the boards: cupping, warping, open joins, blistered paint, and in pine, resin drawn out through the coating. Colours are rated by Light Reflectance Value, LRV, from near 0 for black to near 100 for white. Most suppliers of finger-jointed timber weatherboards set a minimum LRV of 45 per cent for the colours they will stand behind.

Heat-reflective versions of dark colours, such as Resene's CoolColour range, run cooler in the sun and help on the right cladding. But they keep the same LRV as the standard colour, so they do not get you past a weatherboard supplier's limit. Check the LRV against your cladding supplier's limit first. If you want a truly dark house, choose a cladding made for dark finishes at the design stage, not at the repaint. The same caution applies to dark stains on decks and fences.

Stains, powder coat and steel

• Stains and oils show the grain but shield the timber less from UV. Resene's guidance for most stains is selective recoating after two summers, and decks often need it sooner. Recoat the worn areas only.
• Powder-coated aluminium joinery needs washing to keep its warranty. Dulux's New Zealand guidance is at least every 12 months in low-salt urban areas, every six months for beachfront homes, and every three months in a surf zone.
• Pre-painted steel roofing and cladding relies on rain to wash it. The parts rain never reaches, under eaves and spouting, soffits, sheltered garage doors, need washing by hand. For COLORSTEEL, NZ Steel's schedule for those areas runs from every six months in a moderate environment to monthly in the harshest.

Wash with water and a soft brush. Those cycles are written into the warranties, and they are what a manufacturer looks at when a claim arrives. More on that in keeping warranties alive.

Record the exact product

One of the most avoidable failures in exterior paint isn't the paint. It is the touch-up that doesn't match, because nobody wrote down what went on the wall. For every painted surface, record the brand, the product, the sheen, the colour name and code, the batch number on the lid, the date and the painter, and photograph the lid. A colour name alone is not enough: the same colour in a different product or sheen will not match.

That note, with the painter's invoice and the warranty, belongs in your record, where you can find it by paint colour. Then the touch-up in year four matches, and the repaint in year eight starts from the right system.

When to call a painter

Chalking and fine cracks on sound timber need a wash, a light sand and fresh topcoats on the worn walls, usually north and west first. Peeling paint, bare timber or soft wood anywhere needs a painter, and in the soft spots a carpenter before the painter. Flaking paint on a house from the 1980s or earlier needs a painter who works to the lead guidelines. The spring check in wash down and check the paint tells you which of these you are looking at.