Preparing a house for painting

How do you wash a house before painting?

With a brush, a suitable cleaner and low-pressure water. The aim is to lift dirt, salt, mould and chalky residue so the new paint can grip. BRANZ advises against high-pressure water blasting on weatherboards, because it drives water behind the boards. After washing, the walls need to dry out fully.

Mould and moss treatments are chemicals too. Auckland Council asks that they are not washed off without controls in place and kept off the walls if rain is due, so the run-off does not reach a drain.

Scraping and sanding old paint

Loose, flaking or blistered paint comes off with a scraper, back to an edge that is firmly stuck. Sanding then feathers that edge so it does not show through the new coat, and roughens glossy old paint so the next coat bonds. On houses from the 1980s or earlier both steps are done wet, because the old layers are presumed to contain lead.

• Hand scraping suits loose and flaking paint. Well-stuck paint is left in place.
• The state of the old paint is the biggest variable in the price; the cost guide shows how much it moves the published ranges.
• Wire wheels and grinders are not used on house paint; BRANZ says they damage timber and metal.
• Sanding dust is rinsed or wiped off before priming, and collected rather than left to blow.
• Lead-safe methods are explained in full on lead paint in older homes.

Filling cracks, gaps and nail holes

Once surfaces are sound, nail holes, splits and dents are filled, and gaps where boards meet trims are sealed with a paintable flexible sealant. The goal is to stop water getting behind the paint, which is how most exterior paint fails. Filler is sanded flush once it has cured, then primed, because raw filler soaks up paint unevenly.

When is primer needed?

Primer goes on anything bare: timber exposed by scraping, new boards, filler and repairs. BRANZ advises priming raw timber immediately after sanding, before the surface starts to weather. Sound old paint that has been cleaned and keyed usually goes straight to the finish coats. A good quote names the primer for each surface.

To find out what the existing paint is, BRANZ describes a simple check: rub a small hidden patch with a cotton ball soaked in methylated spirits. If colour comes off, the old paint is water-based; if not, it is solvent-based. That decides which primer will bond to it.

Keeping paint and wash water out of the drains

Stormwater drains in Auckland carry whatever enters them directly into creeks and the harbour. Auckland Council’s paint guidance says drains are covered before work starts, water-based brushes are washed into a sink that drains to the sewer (not a septic tank) or onto a lawn or gravel, solvents never go down any drain, and sanding dust and chips are swept into a bin.

The council’s guidance holds landowners to account for what contractors do on their property, so it is worth asking any painter how they will handle wash water. Spills can be reported to the council’s Pollution Response Hotline on 09 377 3107.

How to prep a house for painting yourself

Follow the same order a painter would: wash, let it dry, test the old paint, scrape and sand, fill, then spot-prime. Most paint failures are adhesion failures, where new paint sits on dirt, salt or a loose old coat. Taking time over the dull parts is what makes the finish coats last through Auckland’s sun and moisture.

• Scrub with a soft-bristled broom and a house-wash solution, hose it off, and let the boards dry fully.
• Tape test any doubtful areas; scrape back what lifts to a firm edge.
• Sand edges smooth so they do not show through. On a house from the 1980s or earlier, keep it wet.
• Fill nail holes and cracks with an exterior filler and seal gaps at trims.
• Prime bare timber and filler the same day. Do not leave bare wood out in the weather.

Asked before a repaint

How long does prep take before painting a house?

Usually longer than the painting. On a weatherboard house in fair condition, washing, drying, scraping, sanding, filling and spot-priming can take more of the job than the finish coats. A house that has gone many years without paint needs more. A written quote should show the prep days separately.

Do you need to wash a house before painting?

Yes. Paint does not hold onto dirt, salt, chalky old paint or mould. BRANZ recommends washing exterior paintwork every year anyway. Before a repaint the house is washed with a brush and a suitable cleaner at low pressure, then left to dry fully before any primer goes on.

Do I need to prime before painting?

Bare timber, new filler and any patch sanded through to the substrate need a primer, and BRANZ says it should go on straight after sanding. Sound old paint that has been cleaned and lightly sanded usually goes straight to finish coats. A primer-undercoat also helps when the new paint type differs from the old.

Where should paint wash water go?

Never into a stormwater drain. Auckland Council asks for water-based brushes and rollers to be washed in a sink that drains to the sewer, or onto a lawn or gravel. Solvent-based thinners must not go down any drain. Drains near the work are covered before it starts.

Can you paint over old exterior paint?

Yes, provided the old coat is still firmly attached. It needs a wash, a light sand to dull the surface and feather chipped edges, and spot-priming on bare patches. The tape test tells you whether it is holding: if flakes lift with the tape, that area is scraped back first. On a house from the 1980s or earlier, do the sanding wet.