| Fungal / black growth | Dark patches in damp corners, musty smell | Persistent moisture, poor ventilation | Kill the moisture; treat; anti-fungal coat system |
The Fixes in Detail
Peeling and adhesion failure
Peeling is never a paint problem; it is a bonding problem. The film lifted because the layer under it let go, usually dust left on the wall, a skipped primer, or moisture behind the coat.
Fix, in order: strip everything back to a sound layer (scraper, then wire brush, sand the edges of what remains so the repair feathers in). Let the wall dry fully, the overnight polythene test applies: tape clear plastic to the wall; condensation by morning means wait. Wash, or at least dust, the surface. Prime. Recoat with the full, correct system. Repainting over loose edges without stripping always returns, the new coat lifts the old one from the edge inward.
Bubbling and moisture stains
Blisters mean water is (or was) moving through the wall while the film was trying to hold. Find the source first: leaking pipes or roof, splash zones without protection, ground moisture wicking up, or painting before new plaster had dried (allow about 28 days).
The rule: no refinishing over an active moisture source. Repairs to the coating before the water is stopped are decoration on a timer. Once the source is fixed and the wall dries, treat like peeling, strip blistered areas, prime, redo.
Efflorescence: the salt that comes back
White crystals keep returning because the salts are not the problem; the water carrying them is. Brush the salts off dry (washing drives them back in). Stop the water, rising damp at the base, penetrating rain on exteriors, then allow the wall to dry through. Only then coat, and never seal efflorescence under a film: the salt pressure will push the new coating off.
Map cracking: cosmetic, but permanent if ignored
Shrinkage cracks in a grid pattern come from thick single-pass coats or fast drying. Shallow ones are cosmetic: rake them out with a scraper to give the filler a key, fill with a thin flexible filler, sand flush, and spot-prime. Cracks wider than about 1 to 2 mm, or ones that return along the same line, are structural movement, not shrinkage, they need a crack-repair treatment (V-groove, filler, possibly reinforcement tape) and, if recurring, a look at what is moving.
Hollow patches
A coating that drums under a tap was applied over a weak or dusty base and has already let go, it is only holding on by habit. Cut the hollow area back to sound plaster, key the edges, dampen, and re-plaster the patch before finishing over it. Burying a hollow patch under more coats just schedules a larger repair.
Patchy sheen and pinholes
Both are system-sequence defects, not material defects. Patchy sheen means suction was not equalised, prime, then give the full wall two even coats; patching sheen with a third local coat makes the map worse before it makes it better. Pinholes mean the putty pass was skipped, putty, fine sand, dust, prime, then continue.
Fungal growth
Black or green patches grow where moisture persists. Treat the cause (leaks, condensation, no ventilation), kill the growth with a fungicidal wash, and only then recoat, ideally with a system suited to damp-prone areas. Painting over live fungus produces a film that peels as the colony keeps eating underneath.
Movement Cracks vs Moisture Cracks: the Hairline Distinction
Hairline cracks get a section of their own because they are the defect most often misread. Two different diseases produce a crack that looks identical at arm's length:
Shrinkage cracks come from thick coats or fast drying. They are shallow, sit in a scattered map pattern, do not follow a line from floor to ceiling, and, the key test, do not widen seasonally. They are cosmetic, stable, and fixed once by raking, filling, spot-priming.
Movement cracks come from structural movement: thermal cycling on exteriors, foundation settlement, or a lintel doing overtime. They are straighter, often run diagonally from openings (the classic settlement signature, a diagonal crack rising from the corner of a door or window), and, the key test, they open and close with the seasons. Fill a movement crack with rigid filler and watch the same line return: the wall is moving, and the filler is a diary entry, not a repair.
The treatment is different in kind, not degree: shrinkage gets cosmetic filling; movement gets a flexible system (V-groove, flexible exterior-grade filler or sealant, possibly reinforcement, and on real structural signs, an engineer's opinion before decoration). The seasonal test costs one year of watching, or you can short-cut it by measuring the crack width in the driest weeks and again at the peak of the rains, a width that changes is a verdict.
Efflorescence vs mould deserves the same care: white powdery crystals are salts carried by water (diagnose the water, brush the salts dry, never seal wet); black or green patches are growth on a persistently damp surface (kill it, dry the space, ventilate, then coat). Applying "damp treatment" to either without deciding which it is produces the wrong chemistry and a repeat visit.
What a Defect Costs: the Penalty Table
Every defect has a cheap moment and an expensive moment, and the gap between them is the argument for the whole inspection habit:
| Defect | Cost at skim stage | Cost after second topcoat |
|---|---|---|
| Ridge left in base coat | 5 minutes of sanding | Localized re-skim, feather, prime, blended recoat of the wall plane |
| Pinholes after skim | One putty pass | Putty, sand, prime, two full coats again |
| Skim over hollow patch | Cut, re-plaster, re-skim locally | The patch drums and lifts with the paint attached, cut, re-plaster, re-skim, prime, repaint the wall |
| Painting over damp | Wait for drying | Blisters within the season; strip, dry, prime, redo |
| Unfixed water source | Fix the pipe | The above, on repeat, every season |
Read down the right-hand column: every entry is the left-hand entry plus the price of two coats of paint and the labour of discovery. The torch, the polythene sheet, and the straightedge are the cheapest tools on any site precisely because they move defects from the right column to the left.
Two Case Studies: the Same Symptom, Different Stories
Case 1: the peeling bedroom. A bedroom's paint peels in sheets eighteen months after finishing. The story: the blockwork was plastered late in the project, and the finishing was compressed into the last week before handover, skim over plaster that was eleven days old, not twenty-eight. The trapped construction moisture worked through the skim, the primer never bonded to a damp surface, and the film was doomed before the first coat dried. The fix was the whole sequence again on one wall; the lesson is that the calendar is a defect source: the polythene test costs one night, and skipping it cost a full re-finish of the room.
Case 2: the corner that kept staining. Yellow-brown stains return in the same corner after each repaint. Two painters before had "treated the damp", one with a damp-sealing paint, one with extra coats. The story the stains were telling: a downpipe joint above that corner had been dripping into the wall for years, and the stain was the exit point of a long water path. Fixing the pipe joint, drying the wall, and then refinishing lasted. Neither repaint could have lasted, because both were decoration on an active water route.
Both cases carry the same warning: the defect was readable, in the calendar and in the pipe, respectively, before any coating was opened. Diagnosis is cheap; the repair of an undiagnosed defect is the same repair plus the cost of the coatings that failed over it.
Damp Diagnosis: Getting the Water Story Right
Because moisture drives the majority of finishing failures, the water diagnosis deserves its own method. Three sources, three signatures:
Rising damp. Tide-mark staining starting at floor level, worse in the wet season, often with salt deposits. The water climbs the wall from the ground, failed or missing damp-proof course, poor site drainage, or earth stacked against the wall outside. Signature: it fades as you go up the wall, and returns after each attempt to seal it locally. The fix is at the wall base or the ground, never at the stain.
Penetrating damp. Stains aligned with an exterior feature, under a window, below a roof edge, beside a leaking pipe chase. Water enters from a specific outside fault. Signature: the stain maps to a fault; dry weather shrinks it, wet weather feeds it. The fix is the fault: the flashing, the sealant, the pipe. Refinishing before the fault is fixed is a paint warranty, not a repair.
Condensation and construction moisture. Blotchy damp in corners and behind furniture, or the general moisture of a fresh building still drying out. Signature: no exterior fault, no tide marks; appears in cold corners and seasons with the humidity. The fix is ventilation and time, and the polythene test before coating, walls that are merely drying need days, walls with a water source need plumbing.
One rule closes the diagnosis: if you cannot name the water's path, do not coat over the water's exit. Every "damp-sealing paint success" is a story where the water stopped for another reason, or a story that has not finished yet.
Repair or Repaint?: the Decision Framework
Not every defect needs the full strip-and-redo; the decision follows the depth of the failure:
| Finding | Response |
|---|---|
| Surface defect only (one blister, one scrape, one small stain) with sound coating around it and a named, fixed cause | Localized repair: sand out, treat the cause's evidence, spot-prime, feather in two thin local coats |
| Multiple blisters, patchy adhesion, or any peeling in sheets | Strip the affected wall to a sound layer; full sequence again |
| Pinholes and sheen patching on otherwise sound film | Putty pass is not needed, light sand, dust, one full seal-and-recoat |
| Any defect with no nameable cause | Stop. Diagnose before spending. Repair without diagnosis is the most expensive kind |
| Defect returns after a correct repair | The cause was not what you thought, re-diagnose, especially for water |
The economics are consistent: a localized repair with the right diagnosis is minutes; a localized repair with the wrong diagnosis is the first instalment of a full repaint, because the second and third instalments are already scheduled by the water or the movement that nobody found.
The Defect Audit: a Room-by-Room Checklist
Before refinishing any existing space, walk it with this audit (it takes fifteen minutes and rewrites the budget honestly):
- Tape-test the paint in five places per room (especially near windows, corners, and below previous repairs).
- Tap-test walls for hollow-sounding patches; mark them.
- Sight along each wall with raking light for ripple, ridges, and old repair shadows.
- Look for stains and map each to a possible water source, check the roof edge, pipes, and exterior ground level outside that wall.
- Check skirting and ceiling junctions for gaps and historic movement.
- Test external walls' inside faces for cold-season condensation behaviour if you can.
The audit's output is the work map: walls that need stripping, walls that need spot repair, walls that pass, and the water faults to fix before any coating. Quotes written from that map do not contain surprises, and surprises are where finishing budgets actually go.
The Prevention List: Cheaper Than Any Repair
- Test moisture before coating anything. The overnight polythene test costs nothing and prevents the most expensive defects on this page.
- Prime, always, everywhere. Nearly every adhesion and sheen defect traces to a skipped primer.
- Coats thin, twice. Thick coats are the single source of map cracking, and sanding cannot undo what shrinkage has already cracked through.
- Dust between every stage. Dust is the invisible separator between coats.
- Raking-light check between stages. Every defect on this page is cheaper to catch at the skim stage than after the second topcoat.
Quick Answers to the Defect Questions Owners Ask
"The painter offered a damp-sealing paint for the stained wall, does it work?" Damp-sealing paints hold back cosmetic staining on dry walls and slow vapour; they do not hold back liquid water under pressure from an active source. If the source is fixed and the wall has dried, they are a reasonable belt-and-braces. If the source is live, they blister on schedule, the water is still winning, just behind a more expensive film.
"Can I paint over hairline cracks instead of filling them?" Hairline shrinkage cracks can be filled with a thin flexible filler and spot-primed, but painting over open cracks leaves a shadow map under sheen that grows with every coat. Fill first, it is a fraction of an hour per wall.
"Why does the same defect come back every year?" Because the repair addressed the exit point, not the source. Returning defects are the wall's insistence that the diagnosis was wrong. Re-run the damp diagnosis and the movement checks before spending again.
"Is it worth repairing before repainting the whole house, or just repaint?" Depends entirely on the audit: a sound wall with tired colour is a repaint; any wall with adhesion failure, moisture staining, or hollow patches will lift the new paint with the old. The audit tells you which walls are which, never guess this, because a repaint over failing walls is the most expensive way to discover them.
"Who pays when the defect appears after the contractor left?" The contract does, which is why the scope should name preparation stages in writing (drying time, primer, putty, sanding) and the inspection points. Disputes about finishing are almost always disputes about skipped invisible stages, and both sides lose them.
Conclusion
Defects are the wall's way of reporting what was skipped, moisture testing, primer, thin coats, dust control. Diagnose before treating, never finish over an active water source, and let the raking light audit each stage. For the material quantities a repair or full refinish will take, run the finishing estimator; for the full application sequence that prevents these defects, see the complete guide to interior wall finishing.