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Why Paint Bubbles, Peels, Cracks and Chalks

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09 Nov

Paint fails in a handful of distinct ways, and each one looks different because each one has a different cause. Learning to read them saves money, because almost none of these problems are solved by another coat of paint.

Here is what each failure is telling you.

Bubbling and blistering

Bubbles mean something got between the paint and the surface. Pop one and look inside, because the answer is in there.

  • Water inside the bubble. Moisture is coming from behind: a leak, condensation, or rising damp through masonry
  • Bare substrate inside, no water. The paint never bonded. The surface was damp, dusty, chalky or greasy when it was coated
  • Previous paint inside. The new coat bonded to the old one, and the old one let go from whatever is beneath it
  • Bubbles that appeared within hours on a hot day. Solvent blistering. The surface skinned over in the heat while the paint underneath was still wet, and the trapped solvent pushed through. Our notes on painting in hot weather explain why

Coating over bubbling without finding the cause is the most common wasted repaint there is.

Peeling and flaking in sheets

Large sheets lifting cleanly is adhesion failure, and it is nearly always preparation.

The usual causes are painting over a glossy surface without sanding or deglossing, applying water-based paint over an old oil-based enamel with no bonding primer, coating over chalk on an exterior wall, or painting a surface that was still damp.

If the paint lifts and the surface underneath is smooth and shiny, it was never keyed. If the back of the flake has a powdery residue, it was chalk. For exteriors specifically, our guide to why exterior paint peels goes into it further.

Alligatoring

A pattern of wide cracks across the surface like crocodile skin, usually on old enamel and most often on doors, window frames and weatherboard.

It happens when a hard, brittle film sits over a softer or still-curing one, or when enamel has been built up too thick over decades. The two layers move at different rates and the top one splits.

There is no coating fix. The film has to come back to a sound surface, which on an old door usually means stripping rather than sanding.

Fine crazing

A web of hairline cracks, finer than alligatoring. Usually a topcoat applied before the coat underneath had cured, or applied too thickly in one pass.

On render it can also be the render itself rather than the paint, which is a different problem entirely. See cracks in exterior render.

Chalking

Run your hand down an exterior wall and it comes away with coloured powder.

Ultraviolet light breaks down the binder holding the pigment, and the pigment is left loose on the surface. A degree of it is normal at the end of an exterior coating’s life, particularly on north and west walls. Heavy chalking after only a few years means the product was wrong for the exposure, or it was an interior-grade paint used outside.

Chalk must be washed off completely before recoating. Paint applied over chalk is bonded to loose powder, and it will peel.

Patchy sheen and flashing

The colour is right but the wall looks blotchy in raking light, with duller and shinier areas.

This is porosity. Patched areas, filler, new plaster and old paint all absorb differently, so the topcoat sits differently on each. The fix is a sealer or primer across the whole surface before the topcoats, not a third coat on top.

It also happens when cutting in is done hours before rolling, so the edges dry at a different rate. On a large wall that shows as a visible frame around the perimeter.

Brown or tan streaks on fresh paint

Most common in bathrooms and on newly painted exteriors caught by rain. Water-soluble ingredients migrate to the surface while the film is still curing and leave glossy tan runs.

It looks alarming and it is usually harmless. Wash it off gently with water once the paint has cured. It is a sign the room was too humid or the surface got wet too soon, not that the paint is faulty.

Yellowing white trim

White oil-based enamel yellows over time, and it yellows fastest in rooms that get little natural light: inside wardrobes, behind doors, in hallways.

This is a property of alkyd resin, not a defect and not dirt. If it matters to you, the answer is a water-based enamel, which holds white far better. It is the main reason most trim work in Melbourne has moved to waterborne products.

Rust spots and brown bleed

Small rust-coloured dots pushing through usually mean uncoated fixings underneath: nail heads, screws or steel lintels. They need spot-priming with a rust-inhibiting primer before recoating, or they keep coming back.

On timber, broad reddish-brown staining is tannin, most often from merbau or other hardwoods, and it needs a stain-blocking primer rather than more topcoat.

Mould, or just dirt?

Dab a little diluted bleach on a small spot and wait a minute. If it lightens dramatically, it is mould. If nothing changes, it is dirt or staining.

The treatments are completely different, and painting over mould never works. See mould-resistant paint and wet areas.

White crystalline deposits

Not a paint defect at all. Salts carried out of masonry by moisture, which means moisture is moving through the wall. Our guide to efflorescence covers what to do, and if a leak is involved, water damage repaints.

The thing they have in common

Nine of the ten failures above are preparation or moisture, not product. The paint is almost never the problem, which is why buying a more expensive tin rarely fixes a recurring failure.

Diagnose first, fix the cause, then coat. In that order it lasts. In any other order you are paying to cover the same problem twice.

What we do

  • Diagnose the failure before quoting, rather than quoting a repaint and discovering it later
  • Moisture testing where bubbling or peeling suggests water
  • Proper surface preparation, including deglossing, chalk removal and bonding primers
  • Stain-blocking and rust-inhibiting primers where needed
  • Honest advice when the answer is a repair rather than a repaint

We are based in Richmond and work across Melbourne, the bayside suburbs and coastal Victoria.

Call 0404 330 332. A close-up photo of the failure usually tells us most of what we need before we visit.

📍 Our Service Areas Across Melbourne, the Mornington Peninsula, Surf Coast & Phillip Island

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