Surface Preparation, and Why It Decides How Long Paint Lasts

Paint fails at the surface underneath it far more often than it fails on its own. A quality topcoat applied over a poorly prepared wall will not outlast a mid-range topcoat applied over a properly prepared one. That is the whole argument for preparation, and it is why two quotes for the same house can differ so much.

Here is what each stage does, and why skipping it costs you years.

Washing

Every exterior surface carries a film of dirt, and in Wellington it also carries salt on the coastal suburbs and chalk from the breakdown of the old coating. Paint applied over that is bonding to the contamination rather than to the substrate.

Mould and lichen are a separate problem. They are living, and they grow back through a new coat if they are only painted over. South-facing and shaded walls, and anything under trees, usually need treating rather than just rinsing.

After washing, the building has to dry. On a damp week that can take longer than the washing did, and it is not a stage worth rushing. Moisture trapped under a new coating is a common cause of blistering.

Scraping and sanding

Loose, flaking and lifting paint has already failed. Anything new applied over it is only attached as well as the layer beneath, which is to say not at all. Scraping takes it back to sound material, and sanding feathers the edges so the transition does not telegraph through the finish.

Sanding also gives a key on hard, glossy or chalked surfaces. A new coat over an untouched gloss has very little to grip.

One important caution: on homes built before about 1980, older paint layers may contain lead, and dry sanding or scraping them releases it. That needs proper controls rather than a dust mask. We have a separate note on identifying and dealing with lead paint.

Repairs

This is where a repaint stops being decorating. Rotten timber, split weatherboards, failed putty in window joinery and cracked cladding all need fixing before paint goes anywhere near them. Painting over rot hides it and keeps it wet.

Repairs are also the most common source of variations, because you cannot always see the extent until the old coating is off. A quote should carry an allowance for this and a rate beyond it.

Sealant

The flexible joints around joinery, penetrations and junctions do the actual weatherproofing. Sealant has a shorter life than paint, and by the time an exterior is due for a repaint a good proportion of it has usually gone hard, cracked or pulled away.

Replacing failed sealant during the repaint is one of the highest-value items in the whole job, because those joints are where wind-driven rain gets in. Wellington produces plenty of wind-driven rain.

Priming and sealing

Every bare, filled, repaired or previously unpainted surface needs the right primer. Bare timber, patched areas, galvanised metal and fibre cement all want different products, and a general undercoat is not a substitute for the correct one.

Primers do two jobs: they bond to the substrate and give the topcoat something consistent to sit on, and on timber they help block staining from tannins and knots. A quote with no priming line on a house with bare patches is a quote that has left something out.

How to tell whether a quote includes enough

Look for washing and mould treatment named separately, an allowance for scraping and filling with a rate beyond it, sealant replacement mentioned explicitly, and priming specified by surface type. A quote that lists two coats and nothing else is describing about a third of the work.

You can also ask how long the preparation is expected to take relative to the painting. On an older Wellington home it is often the larger share of the job.

What good preparation buys you

A repaint that reaches the upper end of its expected life instead of the lower end, an exterior that stays weathertight, and the ability to do a straightforward recoat next time rather than starting again from bare substrate. The difference compounds over two or three repaint cycles.

If you would like a written specification showing exactly what preparation your house needs, get in touch, or read more about our approach to exterior house painting.