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Mildew treatment before painting

Updated: 23 October 2026

You are about to roll emulsion over a wall that showed black specks last month, and the last coat failed in the same corner. Mildew treatment before painting is the sequence between a dead, dry surface and the next tin. It is not the cleaning method, and it does not rank anti-mould paints. Paint over live growth stains or peels. Residue from a wash can discolour the new coat even when the mould is gone. Listing prices are on the [mildew treatment comparison](/hub/paints-and-varnishes/mildew-treatment). Peter F. updated this page on 23 October 2026. Our amazon.co.uk links may earn a commission at no extra cost to you.

You are about to roll emulsion over a wall that showed black specks last month, and the last coat failed in the same corner. Mildew treatment before painting is the sequence between a dead, dry surface and the next tin.

Our top picks

Mold Armor Mold Armor FG516 mould blocker 946 ml

Best overall

Mold Armor Mold Armor FG516 mould blocker 946 ml
8.2/10about £25.47
View offer
Professional Strength Professional Strength bleach-free mould remover 500 ml

Best value

Professional Strength Professional Strength bleach-free mould remover 500 ml
8.0/10about £9.99
View offer
Strewth Strewth mould preventer spray 500 ml

Alternative

Strewth Strewth mould preventer spray 500 ml
6.6/10about £14.99
View offer
On this page
  1. 1.Why the stain has to be dead first
  2. 2.Rinse, then dry
  3. 3.What you coat with
  4. 4.Which bottle in this set is a pre-paint wash

Why the stain has to be dead first

Emulsion is a colour, not a biocide. Dulux Trade's specifier note says painting over mould, even with a mould-resistant paint, is pointless if the moisture is still there. The same maker's FAQ says eradicate the mould before you paint. GOV.UK's housing guidance makes the same split: use a mould and mildew product rather than bleach, dry the surface, and fix the cause. A bleach mix that only pales the colour is not the method here. The clean itself is how to use mildew treatment.

Rinse, then dry

The wait belongs to the wash you used. Dulux's consumer page for Weathershield Fungicidal Wash says leave it until dry, for a minimum of one hour, then rinse and scrub so the next coat is not discoloured. Work above 10°C. Dulux Trade's FAQ says leave 24 hours, wash it off, let the wall dry, then use a mould-inhibiting paint. Follow the tin in your hand. Do not give Professional Strength or Strewth a 24-hour rule their amazon text does not state. None of the three kept listings prints a dry-before-paint time, an HSE number, or coverage.
Mold Armor Mold Armor FG516 mould blocker 946 ml

Testsieger

Mold Armor Mold Armor FG516 mould blocker 946 ml
8.2/10about £25.47
View offer

What you coat with

Once the wall is clean and dry, Dulux points decorators toward a mould-inhibiting system: wash, then a stain-block primer where the mark has bled, then a fungicidal topcoat. Ordinary emulsion on a dirty or damp wall is not anti-mould paint. An NHS leaflet circulated by Spring Hill Medical Centre says kill mould with a spray that has an HSE number, then use fungicidal paint, and do not paint or wallpaper over that paint. Healthy Homes gives the same overcoat warning. This page does not review those tins. They are compared under anti-mould paint. A stain-block primer hides a water mark. It does not dry a failed gutter. GOV.UK suggests looking again at least six weeks after remedial work.

Which bottle in this set is a pre-paint wash

Only a listing that names walls or masonry is even a candidate, and none of the three is a documented pre-paint system. Professional Strength's title names bathrooms, kitchens, and walls, and the bullets add masonry and wood. That is the closest match for a small patch. It does not state hours before emulsion, coverage, or an HSE number. At £9.99 for 500 ml it is a patch bottle. Matching a bottle to a wall or to fabric is the mildew treatment buying guide. Strewth's bullets say spray on and leave, with no wipe. A leave-on film is the opposite of a rinse-before-paint wash unless the label you are holding says otherwise. Mold Armor FG516 names fabric, furniture, and upholstery. It is the wrong job before emulsion. If you need an HSE number on the wash, Bartoline's maker page prints HSE 9626, and a four-litre datasheet on the same site prints HSE 10392. Do not copy either number onto these ASINs. The label wins.

Common questions

Can I paint straight over the black stain?
No. Live growth under emulsion fails, and Dulux Trade says eradicate mould before paint. Kill and remove the growth, rinse if the wash label says so, dry the wall, then paint. A stain that has only been bleached paler is not a dead surface.
Do I have to rinse the wash?
Rinse if the label says leftover product will discolour the next coat. Dulux's consumer page says rinse and scrub after a minimum of one hour. The trade FAQ says leave 24 hours and wash off. Do not assign that rinse to Professional Strength or Strewth. Their listings do not state it.
How long before emulsion?
As long as the wash label says, then until the wall is dry. Dulux prints a one-hour minimum on the consumer page and 24 hours on the trade advice. None of the three amazon.co.uk listings states a dry-before-paint time. Painting the same afternoon onto a damp wall is the common failure.
Will anti-mould paint work if I skip the wash?
It can look fine until moisture and remaining growth push through. Dulux's specifier page says painting over mould, including with a mould-resistant paint, without fixing the moisture is pointless. Wash, then a stain-block primer if the mark has bled, then a fungicidal topcoat.
Can I put ordinary emulsion over a fungicidal paint?
Not if that paint's instructions forbid it. The Spring Hill leaflet and Healthy Homes both say do not paint or wallpaper over fungicidal paint. Ordinary emulsion can seal the biocide away and still fail at a damp corner.
Which of the three bottles is a pre-paint product?
Professional Strength is the title that leads with walls, bathrooms, and kitchens, and its bullets mention masonry. It still has no printed wait, coverage, or HSE number, so it is a candidate for a small patch, not a specified wash. Strewth is a leave-on preventer. Mold Armor's listing is fabric.
What if the stain is inside the plasterboard?
If the board is soft and the growth is inside it, washing the face will not hold. Permagard treats that as a replacement once the leak is fixed, then a wash of the sound surround before new plaster and paint. A stain-block primer is for sound plaster, not a spongy sheet.

Related pages

Mildew treatment comparison

How to use mildew treatment

Anti-mould paint

Mildew treatment buying guide

Bathroom mildew treatment: ceiling, grout and seals

Best mildew treatment: which bottle fits your own job

Sources

Public sources from the research digest, last checked on 22 October 2026. Shelf prices and star lines are the amazon.co.uk figures stored on 19 October 2026. The price band is this set, not a market average. No listing was lab-tested.

  1. GOV.UK: damp and mould, health risks for rented housing (retrieved 22 October 2026)
  2. GOV.UK: Awaab’s Law guidance for tenants in social housing (retrieved 22 October 2026)
  3. The Hazards in Social Housing (Prescribed Requirements) (England) Regulations 2025 (retrieved 22 October 2026)
  4. GOV.UK: damp and mould in the private rented sector (retrieved 22 October 2026)
  5. Bartoline Fungicidal Wash (retrieved 22 October 2026)
  6. Dulux Weathershield Fungicidal Wash (retrieved 22 October 2026)
  7. Dulux Trade Paint Expert: general FAQs (retrieved 22 October 2026)
  8. Healthy Homes: damp and mould guide (retrieved 22 October 2026)