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What to check, and in what order

My paint is blistering or bubbling

Open one blister and look at what is underneath. Bare substrate means the whole system lost its grip; a previous paint layer means only the top coat did; and moisture inside means water is pushing it off from behind. Three different answers from one cut.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

A blister is paint that has lifted away from what is beneath it, and something got between them or pushed them apart. The useful question is what, and the answer is directly behind the blister.

SO OPEN ONE. Cut a blister with a sharp blade and look at two things: what is exposed underneath, and whether there is any moisture. Bare substrate — timber, plaster, masonry — means the whole paint system has released from the surface, which points at preparation or at moisture behind. A previous coat of paint still attached means only the top layer failed, which points at what was applied over what, or at the condition of that older coat.

MOISTURE is the other axis and it is often visible. A blister that contains liquid, or a damp substrate underneath, is being driven from behind: water moving through the wall or the timber and lifting the film as it tries to leave. That is a building problem showing up as a decorating one, and repainting it repeats the failure at the same speed.

The remaining causes are about what happened at application: painting in direct sun or on a hot surface, so the surface skinned before the solvent left; painting over a contaminated or glossy surface; or trapping moisture by coating a substrate that was not dry.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. Moisture behind the paint

    Most common

    Liquid inside the blister or damp substrate beneath it, often in a patch rather than across the wall, and recurring in the same place after repainting.

    How to check: Cut a blister and feel the substrate. Damp underneath means the source is behind the paint, not in it.

  2. Painted over a surface that was not clean or keyed

    Most common

    Blisters exposing BARE substrate, often over a glossy or previously contaminated surface, with the back of the film smooth and clean.

    How to check: Look at the back of the lifted film. Smooth and clean means it never bonded; dusty means the surface was not clean.

  3. Painted in direct sun or onto a hot surface

    Common

    Blisters on the elevations that get most sun, appearing within hours or days of painting, and usually only in the top coat.

    How to check: Which elevations and when. A south-facing wall blistering the day after painting identifies itself.

  4. An incompatible coating over another

    Common

    The top coat lifting cleanly from an intact coat beneath, often where a water-based paint has gone over an old oil-based one.

    How to check: What was underneath and what went on it. A solvent-based gloss under a water-based emulsion is the classic pairing.

  5. Painted before the substrate had dried

    Common

    Widespread rather than local, on new plaster, new render or recently washed surfaces, appearing within weeks.

    How to check: The dates. New plaster needs weeks rather than days, and coating it early traps the water that then pushes the film off.

  6. Rust or decay under the coating

    Less common

    On metal, blisters with rust beneath and often a stain spreading from them; on timber, soft or discoloured wood under the film.

    How to check: Probe the substrate under an opened blister. Soft timber or rust means the problem is the material, not the paint.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the entire diagnosis for the two largest families and costs one blister that had already failed. Bare substrate, an intact previous coat, or moisture — three different answers.

  2. 2

    Clean and smooth means it never bonded; dusty means the surface was not clean; carrying substrate with it means the substrate failed rather than the paint.

  3. 3

    It separates a decorating problem from a building one, and it is the single finding that makes repainting pointless until something else is fixed.

  4. 4

    Sun-facing walls blistering immediately after painting is an application-conditions problem; a patch on one wall appearing months later is moisture.

  5. 5

    Incompatible systems and glossy surfaces are among the most common causes and are settled by a question rather than a test.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Paint Calculator →Vapor Barrier Dew-Point Condensation Risk Calculator →Gypsum Plaster Finish Coat Volume Calculator →

When to stop and get somebody in

  • Where the substrate under a blister is persistently damp, since the water is arriving from somewhere and finding that matters more than the decoration.
  • Where blistering keeps returning in the same place after correct preparation and repainting — the cause is behind the surface.
  • Where timber under the coating is soft to a probe, which is decay rather than a coating failure and may extend well beyond the blistered area.
  • Where the paint being disturbed is on a building old enough that its earlier coatings may contain lead, in which case disturbing it is a controlled activity rather than a scrape.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.