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Remove mould & stains before paint.
Treated, killed and stain-blocked — so the new paint doesn't bloom through in 6 months.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Top tips
Hiring a decorator, without the regret.
Four moves that separate a smooth job from a nightmare.
Bleach instead of biocide
Bleach kills surface mould but doesn't kill spores in the substrate. Mould returns within months.
Ignoring underlying cause
Condensation, leak or cold-bridging causes mould. Address the cause first, treat the surface second.
Water stains bleeding through
Untreated water stains bleed through any number of paint coats. Always seal with proper stain-block primer.
Cheap stain block
Watered-down emulsion isn't a stain block. Use Zinsser BIN (shellac-based) for serious stains.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Mould biocide spray treatmentPer small area
£15–£60
Stain block primer per areaBIN, Coverstain
£20–£80
Mould treatment small bathroomIncluding biocide and primer
£60–£150
Bedroom mould treatmentUsually condensation patches
£80–£250
Living room water stainStain block treatment
£40–£150
Investigate underlying dampSurvey, may need specialist
£60–£300
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Mould and water stains must be properly treated before painting — paint applied over them lets the problem return within months. Expect to pay £40–£300 for UK pre-paint mould and stain treatment depending on area affected.
What Does the Work Involve?
For mould: surface cleaned with HG mould remover or Polycell mould treatment. Affected area treated with biocide (kills spores in the substrate, not just the visible mould). Underlying cause investigated — usually condensation, leak or cold spot. Stain-block primer (Zinsser BIN, Coverstain) applied to seal in any remaining stain. Two top coats then applied normally. For pure water stains: stain-block primer first, no biocide needed.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Mould biocide spray treatment | 15 | 60 | Per small area |
| Stain block primer per area | 20 | 80 | BIN, Coverstain |
| Mould treatment small bathroom | 60 | 150 | Including biocide and primer |
| Bedroom mould treatment | 80 | 250 | Usually condensation patches |
| Living room water stain | 40 | 150 | Stain block treatment |
| Investigate underlying damp | 60 | 300 | Survey, may need specialist |
| Anti-mould paint upgrade | +£40 | +£100 | vs standard top coat |
How Long Does It Take?
Mould treatment: 1–2 hours, plus drying. Stain block primer: 4–6 hours dry time. Full decoration cycle: 2–3 days.
DIY or Professional?
Surface mould treatment is DIY-able. Persistent damp problems require a professional damp specialist or builder to identify the cause. Painting alone over recurring mould is wasted money.
Choosing the Right Tradesperson
A decorator for surface treatment; a damp specialist for underlying causes. Anti-mould paints (Polycell, Crown Trade Clean Extreme) are worth the premium in bathrooms and on cold walls.
UK Regulations
No specific Building Regs for mould treatment. Landlord regulations (Awaab's Law from 2026) require landlords to address mould within strict timeframes — paint-over solutions are no longer compliant.
Common Problems
- Bleach instead of biocide: Bleach kills surface mould but doesn't kill spores in the substrate. Mould returns within months.
- Ignoring underlying cause: Condensation, leak or cold-bridging causes mould. Address the cause first, treat the surface second.
- Water stains bleeding through: Untreated water stains bleed through any number of paint coats. Always seal with proper stain-block primer.
- Cheap stain block: Watered-down emulsion isn't a stain block. Use Zinsser BIN (shellac-based) for serious stains.
Frequently Asked Questions
Why does mould keep returning?
The underlying cause hasn't been fixed. Mould grows on cold or damp surfaces — usually condensation (ventilation issue) or leak (plumbing/roof). Treat the cause, not the symptom.
Does bleach kill mould?
Surface mould yes; spores in the substrate often not. Use proper biocide (Polycell, HG, Ronseal Anti-Mould) which kills both.
Can I paint over a water stain?
Only after sealing with stain-block primer. Untreated water stains bleed through any number of top coats.
What's anti-mould paint?
Paint with biocide additives — fungicide/algaecide. Helps prevent mould returning. Not a substitute for fixing the underlying cause.
Stain-block primer brands?
Zinsser BIN (shellac-based, best for serious stains), Zinsser Coverstain (water-based), Polycell Stain Block, Bedec Stain Block.
Should I get a damp survey first?
If mould recurs or covers a large area, yes. A PCA-qualified damp surveyor identifies the cause — usually £80–£250 for a domestic survey.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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