Mould-Defence Additives: What They Do and What They Do Not

Mould-defence additives are widely misunderstood, largely because of how they are marketed. They are useful products with a narrow and specific purpose, and they are frequently bought as a solution to a problem they cannot solve. Being clear about the difference saves both money and a repeat of the same job in eight months.

What the additive actually protects

These products are film preservatives. They are added to a coating so that the dried paint film itself resists colonisation by mould and mildew. That is genuinely useful in a bathroom, a kitchen, a laundry or a north-facing exterior wall, where a paint surface would otherwise host growth.

What they do not do is change the conditions in the room. If moisture is condensing on a cold wall every night, mould will grow on the dust and soap residue that settles on the paint, and a film preservative has no effect on that. It protects the paint; it does not sterilise the room.

Diagnose before you buy

Black spotting in the corners of a bathroom ceiling and along the cold edges of external walls is condensation mould, and the cause is ventilation, insulation and heating. An additive will slow its return on the painted surface, and it will come back.

A tide mark rising from the skirting, with salts on the surface and paint lifting, is rising or penetrating damp. Coating over it — with or without an additive — traps the moisture and makes it worse. That needs the source fixing.

Growth on one small area next to a leaking pipe or a failed window seal is a leak. Fix the leak.

The honest test is simple: if you can explain where the water is coming from, deal with the water first.

Cleaning before coating

Painting over existing mould does not kill it and does not hide it for long — it grows through. The surface has to be cleaned and treated with a proprietary mould remover, following the contact time on the label, then rinsed and dried thoroughly.

Wear eye protection and gloves, ventilate the room, and never mix a mould treatment with any other cleaning product. The combination of a chlorine-based product and an ammonia-based one produces a genuinely dangerous gas, and it is an accident that happens in domestic bathrooms every year.

Using an additive correctly

Additives are dosed by volume of paint, and the ratio on the bottle is not a suggestion. Under-dosing gives no useful protection; over-dosing can affect the film, the sheen and the drying time. Measure rather than pour by eye.

Stir thoroughly and use the batch within the period stated — most are intended to be mixed and used, not mixed and stored for a year. Where a product carries a scent, as some lemongrass-based additives do, that scent is a carrier characteristic rather than the active function, and it fades as the film cures.

What to expect afterwards

A well-prepared, correctly dosed surface in a room with adequate ventilation should stay clean for years. The same surface in a room where wet towels dry every night and the extractor fan does not work will show growth again, and the additive will have bought time rather than a solution.

That is not a failure of the product; it is the boundary of what the product is for, and it is worth knowing before rather than after.

Improving the conditions

• Run an extractor during and for twenty minutes after showering; most fans are switched off far too early.
• Keep a small gap behind furniture on cold external walls so air can move.
• Heat consistently at a low level rather than in short bursts — intermittent heating produces condensation cycles.
• Dry laundry outside or in a ventilated space rather than in a closed room.
• Wipe down tiles and glass after showering; it removes the water that would otherwise evaporate into the room.

Reading the label

Look for three things: the dose ratio, the paint types it is compatible with, and any statement about interior versus exterior use. Some additives are not suitable for use with certain specialty or mineral finishes, and adding one to a lime-based product in particular can interfere with how it cures.

We reproduce the manufacturer’s dose ratio and compatibility notes on each product page in additives and waxes. Where the manufacturer does not state compatibility with a finish you are planning to use, ask us before mixing rather than after.

Choosing the paint as well as the additive

An additive works with the coating it is mixed into, and some coatings are better suited to damp rooms to begin with. A washable satin or eggshell in a bathroom will survive wiping down far better than a chalky matt, which absorbs moisture into its surface and gives growth somewhere to sit.

Where a manufacturer already sells a bathroom or kitchen formulation, it usually contains a film preservative as standard, and adding more does not improve it. Check the tin before buying an additive you may not need.

Recording what was used

Note the product, the additive and the dose on a label inside a cupboard door in the room. When the surface needs touching up in two years, that note answers the questions that otherwise mean guessing — which sheen, which colour, and whether an additive was used at all.

It also matters if the growth comes back. Knowing exactly what was applied, at what rate, is the difference between diagnosing a ventilation problem and blaming a product that was actually working as intended.