Why Anti Mould Paint Keeps Failing in Bathrooms

If you’ve ever been called back to a bathroom you painted a year ago only to find black spots crawling across the ceiling, you know exactly how annoying it is. The customer paid for anti-mould paint. You put it on properly. And somehow, here you are again, scraping it off and recoating for free.

Nine times out of ten, the paint isn’t the problem. It’s what’s happening behind the walls and above the extractor that nobody dealt with the first time. Let’s get into why these coatings keep failing and what you can actually do about it on site.

What Anti-Mould Paint Actually Does

Anti-mould coatings have a biocide built into the paint film that stops mould spores from taking hold on the surface. But that biocide wears out. Most manufacturers say it’ll last two to five years, and in a bathroom that gets steamed up every day, you’ll be lucky to get the full two.

The paint also can’t touch moisture that’s already trapped inside the wall or condensing behind tiles. If the substrate is damp, mould will grow underneath the film and push straight through it. You’ll start seeing bubbling, discolouration, then full patches of growth. No amount of repainting will fix that.

How to Diagnose the Real Problem

Before you even quote a repaint, spend ten minutes working out what’s actually causing it. Start with the extractor fan. Hold a bit of tissue up to it while it’s running. If the tissue doesn’t stick, the fan isn’t pulling enough air. Loads of bathroom fans get clogged with dust over time, or they were too small for the room when they went in.

Next, look at where the mould is worst. If it’s on the ceiling right above the shower, that’s a ventilation problem. If it’s on an external wall, you’re probably dealing with a cold surface where warm, moist bathroom air keeps condensing. If it’s behind the bath panel or around the base of tiles, there’s likely a leak or failed sealant letting water into the wall cavity.

Every one of those needs a different fix. And none of them is another tin of anti-mould emulsion.

How to Have the Conversation

Most customers want the cheapest option. They’ll ask you to wash the mould off and slap two coats of something over it. You can do that, but make sure your quote says clearly that you’re treating the surface only and that the underlying ventilation or damp issue hasn’t been resolved. Put it in writing. That one line on the quote will protect you when the mould comes back in six months.

If they push back, keep it simple: the paint can slow the mould down, but it won’t stop it if the room can’t dry out properly.

What to Specify When They Want It Sorted Properly

For the shower itself, the best long-term answer is to get rid of the painted surface altogether and replace it with something mould can’t feed on. A lot of decorators are now fitting hygienic wall cladding PVC sheets in wet zones for this exact reason. The material is non-porous, so there’s nothing for spores to latch onto. Water just sits on the surface and wipes off. There are no grout lines to worry about, and you won’t need to repaint every couple of years.

Pair that with a properly sized extractor fan, ideally one on a humidistat so it kicks in automatically, and the customer will have a bathroom that won’t need you back for a mould callback.

When Paint Is Still the Right Call

Anti-mould paint does have its place. On a well-ventilated ceiling with no cold bridging issues, it’ll do the job for a few years. It’s the right product for the right situation. The trouble starts when it gets used as the only fix in a room that actually needs better ventilation, insulation, a surface swap around the wet zone, or all of the above. Know the difference and explain it clearly. You’ll save yourself a lot of unpaid return visits.