Should you waterblast a house before painting it?

Usually not, at least not the way most people use a waterblaster. A house needs washing before it is painted, but a high-pressure waterblaster can gouge timber and force water up behind weatherboards, and a house wash applied with a brush and rinsed at low pressure does the cleaning job better.

Waterblasting has its place on hard surfaces such as concrete and paths. On painted timber weatherboards, it is the wrong tool more often than the right one.

What is washing before painting actually for?

New paint has to bond to the surface under it. Three things stop that happening:

• Chalk. As exterior paint ages, the top layer breaks down into a fine powder. Wipe a dark cloth across an old painted wall and you will see it.
• Mould, moss and lichen. Especially on the shaded south side and under eaves.
• Salt, grime and spider webs. Salt matters on coastal and harbour-edge houses, where it builds up on sheltered surfaces the rain does not rinse.

Resene's exterior timber guide says chalk and other contaminants need to be removed, and that the most effective way is a house wash product, scrubbed on the way you would wash a car. It describes that approach as far more effective at de-chalking and cleaning than waterblasting. The aim is a clean, sound surface. Blasting off anything loose is a different job, called preparation, and it is done with scrapers and sandpaper.

How does a waterblaster damage a house?

Resene's problem-solver page on waterblasting says timber can be damaged by waterblasters and that water can be forced up under weatherboards. It also notes that waterblaster pressure varies dramatically and that waterblasting is generally best for firmer surfaces such as concrete and steel.

In practice, the damage looks like this:

• Gouged and furred timber. The jet tears the soft earlywood out from between the harder grain, leaving a ridged, fuzzy surface that will not take paint smoothly. Resene's timber guide specifically notes that cedar, being softer than pine, is easily damaged by waterblasting, and its only recommendation for cedar is to scrub or wash.
• Water driven in behind the cladding. Weatherboards are designed to shed rain falling down them, not a jet aimed up under the laps. Water pushed up behind boards soaks the back of the timber and the framing, and it takes time to dry out.
• Wet timber painted too soon. Timber soaked by blasting needs a proper drying period before painting, and in a Wellington winter that period can stretch. Paint over damp timber and you invite blistering and peeling.
• Paint torn off sound areas. A jet can lift edges of paint that were still well bonded, turning a wash-and-recoat into a bigger preparation job.
• Broken glass and wet joinery. Resene's page lists broken windows and lights among the damage a waterblaster can do when used inappropriately, and the gaps around old sashes let a jet of water straight in.

Does waterblasting kill mould?

No. Resene's waterblasting page says it will not kill moss and mould. It can blast off the visible growth, but what is left alive in the surface grows back, so treat it with a moss and mould killer first, then wash.

What should you use instead?

Method	Good for	Risks
House wash, brush scrub, low-pressure rinse	Chalk, dirt, salt and mould residue on sound paint	Slower; needs a good rinse so no residue is left
Moss and mould treatment first	Killing growth on shaded elevations	Needs time to work before washing, per the label
Garden hose or low-pressure rinse	Rinsing off wash product	Minimal, if not aimed up under the laps
High-pressure waterblasting	Hard surfaces such as concrete and paths	Gouged timber, water behind boards, spreading paint chips
Scraping and sanding	Removing loose or flaking paint	Lead precautions on older houses

A soft brush on an extension pole, a bucket of house wash, and a hose to rinse will do almost every house. It takes longer than blasting, and it is still the better result. If you do use a waterblaster, Resene's timber guide says low pressure can be used but is less effective and can still damage timber. Keep the pressure down, the nozzle well back, and aim downward with the laps, never up into them.

Our exterior painting quotes set out how each house will be cleaned, so you can see the method before any work starts.

What about lead paint?

This is the strongest reason not to blast an old house. Homes painted before the 1980s may contain lead-based paint. Have it tested before sanding or stripping, and before any high-pressure water goes near it. A waterblaster aimed at flaking lead paint sprays chips and contaminated water across lawns, gardens and the neighbours' sections, and contaminated soil is hard to put right. On a positive test, wash gently, collect scrapings on drop sheets, and keep water pressure low. Our guide to lead paint removal covers what else changes.

Is it ever the right call?

Yes, on the right surfaces. Concrete block walls, masonry, paths and driveways can take a waterblaster when it is used with care. Resene's own guidance leans that way: firmer surfaces are where waterblasting belongs.

For a timber house, washing is one step in a sequence that also includes mould treatment, scraping, sanding, filling and priming. How that sequence runs is covered in our exterior painting preparation guide. If you are getting quotes, ask each painter how they plan to clean the house. An answer that starts and ends with a waterblaster is worth a follow-up question.

If you would like a quote that spells out the washing and preparation method for your house, get in touch.