Blistering, peeling or flaking: what each kind of paint failure tells you

Blistering means moisture is trapped under the paint film and pushing it off the surface. Peeling in wide sheets means the coating never bonded properly to what was underneath it, and flaking into small brittle chips means an old film has hardened and lost the flexibility it needs to stay attached.

Those are three separate problems with three separate fixes, and only one of them is really about the paint. Scrape and recoat without working out which one you have and two of the three will be back within a couple of years.

What is actually happening under a blister?

A blister is a dome of paint lifted clear of the surface with a void behind it. Break one open and the answer is usually sitting right there. If you can see bare timber or bare plaster at the bottom of the blister, water has got under the whole system from behind or through a gap. If the blister only goes through the top coat and there is sound paint underneath it, that top coat went on over a hot or damp surface and sealed moisture or solvent in.

On a weatherboard house the usual water sources are a leaking gutter joint, a downpipe discharging against the cladding, a failed seal at a window scriber, or boards sitting close enough to the ground to wick moisture up. Inside, blistering above a shower or across a bathroom ceiling is nearly always ventilation rather than paint, and no product fixes an extractor fan that vents into the ceiling space instead of outside. If the rest of the house is due anyway, that is worth sorting before any interior repaint starts.

Painting over a blister without finding the water is a wasted weekend.

Why does paint peel off in sheets?

Peeling that comes away in wide flexible sheets, sometimes still showing the roller pattern, is an adhesion failure. The film itself is sound. It simply had nothing to hold on to. The causes, roughly in order of how often we find them:

• A gloss or semi-gloss surface recoated without sanding or de-glossing.
• Chalky weathered exterior paint that was not washed, so the new coat bonded to loose powder.
• Waterborne paint applied straight over old solventborne enamel with no bonding primer.
• Dirt, salt spray, mould or mould-killer residue left on the surface.
• Bare timber, plaster or filler painted without being sealed first.

The giveaway is the back of the peeled sheet. If it is clean and smooth, the failure happened at the bond line, and the answer is preparation, not a more expensive paint.

What does flaking or chipping mean?

Flaking is different again. The chips are small, hard and brittle, they snap rather than bend, and the break usually runs through several layers at once. That is an old, thick coating that has gone hard with age until it can no longer move with the timber under it. Weatherboards swell and shrink through every wet and dry cycle, the paint has to stretch with them, and after enough decades it stops being able to.

This shows up constantly on the 1900s to 1930s weatherboard stock around Wellington, where boards can be carrying generations of accumulated coats. Once a coating is that thick and that brittle, spot repairs stop being economic, because every scraped edge becomes the next place it lets go.

What about fine cracking with nothing lifting yet?

There is a fourth thing you will see on a wall, and it is the one worth catching. A coating near the end of its life often shows a fine grid of cracking across the surface, sometimes called crazing, while still being firmly stuck down. Nothing is domed, nothing peels when you push a scraper at it, and from a few metres away the wall reads as tired rather than failed.

That is a film telling you it has lost its flexibility but has not yet let go. Caught at this point the job is a wash, a sand and two coats. Left for a few more winters, the same wall becomes the brittle flaking described above and the preparation bill multiplies.

How do I tell which one I have?

What you see	What it means	What actually fixes it
Domed bubbles, bare substrate visible inside	Water getting in behind the coating	Find and stop the water first, then strip back and rebuild the coating system
Small bubbles, sound paint visible inside	Applied over a hot, damp or freshly filled surface	Scrape the affected area, sand, prime, recoat in better conditions
Large flexible sheets, clean on the back	Adhesion failure, preparation was skipped	Strip the failed area, wash, sand or de-gloss, bonding primer, two coats
Small brittle chips breaking through several layers	Old film has gone hard and lost flexibility	Full strip on the worst elevations, back to a sound base
Fine grid of cracking, nothing lifting yet	Coating is near the end of its life but still attached	Repaint now, before it becomes flaking and doubles the preparation

Does it matter which wall it is on?

It tells you a great deal. South and west walls take the wind-driven rain here, and they will often be peeling or flaking while the sheltered side of the same house still looks presentable. That is exposure, not a paint defect, and it is why an honest quote for an exterior repaint can carry several times more preparation on one elevation than on another.

It also means you should judge the house by its worst wall rather than its best. Plenty of places look tidy from the street and are well past due at the back.

Which of these can I fix myself?

Small isolated damage is genuinely a do-it-yourself job and worth doing. A blistered patch above a shower, a peeling window sill, a metre of flaking on one board: scrape back to a firm edge, feather that edge with abrasive paper, prime the bare area, two coats over it. That is a Saturday.

Stop and get someone in when the failure covers whole elevations, when it keeps returning in the same place, when the timber under it feels soft, or when the house is an older one. WorkSafe's position on lead-based paint is that if a building was built in the 1980s or earlier, it is best to presume it has been painted with lead-based paint. Treat your house that way until a test says otherwise, and keep dry sanding, power sanding and burning off out of it, because those are the methods that put lead dust through the house and garden.

Can you just paint over any of it?

None of the three, and that is the whole reason for telling them apart. Blistering and peeling cannot be usefully coated over, because a new film is only as well attached as the thing beneath it. Flaking is already past the point where another coat holds anything together. Every extra coat on a failing base makes the eventual strip slower and dearer.

The paint you can simply wash, sand and recoat is the stage before all of them: sound but tired, perhaps a little chalky or finely crazed, with nothing lifting anywhere. That is the cheapest repaint you will ever buy, and it is why repainting early costs less than repainting late.

What to do next

Walk each side of the house, break open one blister and pull one loose flake. The back of that flake tells you most of what you need to know. If the affected area is small, deal with it yourself. If it runs across whole walls, or the timber underneath gives under a screwdriver, get eyes on it before you buy paint.

We will come and tell you which of the three you are looking at, and whether it needs doing this summer or can wait a year. Get in touch for a look and a quote.