A gasket that holds water cannot stay clean
Look at the shape before you look at the mould. The door seal is a moulded bellows with two or three lips, and between those lips are U-shaped channels that open upwards. They have to be shaped that way — that is what lets the seal flex as the door closes and lets the whole assembly move with the drum. The consequence is that each channel is a little trough, and each trough catches what the wash throws at it: half-dissolved powder, softener, lint, hair, the tissue nobody found in a pocket. Measurements put 3 to 5 mL of water in every fold after every cycle, and a front-loader can keep 1.2 to 2.4 litres sitting in the outer tub, the pump sump and the bellows combined. That keeps the inside of the machine above 92% relative humidity for up to 48 hours — and mould needs 85%.
So the black dots are not evidence that somebody is dirty. They are what that shape does when the water never gets out. And there is one more piece of the design that decides whether it does get out: a small drain hole at the six o'clock position, the lowest point of the lowest fold. When it is clear, the trough empties into the drum and dries. When lint and limescale plug it — and they do — you have built a reservoir that holds water permanently. That is why a seal that has been cleaned properly comes back in three weeks: nobody cleared the hole.
Two tests, and they settle it without guesswork
Before you buy anything, find out which of three things you have, because the answers are completely different: clean it, fix why it recurs, or replace it.
- The swab test. Put a little mould remover on a cotton bud and hold it against the darkest spot for thirty minutes, then wipe. If the rubber underneath is back to its own colour — pale grey or buff — the growth is on the surface and in the pores, and cleaning will work. If it is still black, the pigment is inside the material and no amount of product will reach it.
- The one-second test. Pinch the blackened rubber, let go, and count. A sound elastomer springs back immediately. One that stays dented, or feels tacky, or has gone hard and shiny, has degraded — the polymer chains are gone and nothing you apply will bring them back. Small cracks deeper than half a millimetre say the same thing.
- The area test. More than a third of the circumference, or black running right around the ring, is past the point where spot cleaning is honest about the result. The seal is now a reservoir with a damaged surface, and it will keep releasing spores into every load.
Two more signals belong to the third case rather than the first: a seal that weeps water onto the floor during a wash, and one that has started to come away from the panel or the drum lip. Both are physical failures, and neither is a cleaning problem.
Why bleach is the wrong tool — and it is measured, not opinion
Every list of washing-machine tips opens with «soak it in bleach overnight». It is the most intuitive idea available: it is cheap, it is what we use on mould in the bathroom, and it visibly works — the black goes away. Then it comes back, and people conclude the mould is unkillable. What actually happened is more specific, and it has two halves.
The first half is that bleach bleaches rather than kills. Sodium hypochlorite oxidises the pigment, so the stain disappears while the hyphae inside the rubber's micropores are still there. You have removed the evidence, not the colony. That is why the same spot reblackens in two to three weeks — and why it feels like the cleaning «didn't work» when in fact nothing was ever killed.
The second half is that it is eating the part. Chlorine is a strong oxidiser and rubber is a polymer chain; repeated exposure breaks the chains. Accelerated ageing tests put it at a 42% loss of tensile strength and a 35% rise in hardness after five contacts with neat bleach, with degradation running roughly three times faster over six months. Some manufacturers go further and void the gasket warranty if bleach has been used. The result is a seal that goes brittle, then micro-cracks, and cracks are the best mould habitat there is — so the well-intentioned fix produces more mould, faster.
What works instead is chemistry that is kinder to the elastomer and harder on the biofilm: oxygen bleach, sodium percarbonate, at 60 °C or above, where it releases hydrogen peroxide and hydroxyl radicals that break up the extracellular matrix the colony hides inside. And the order matters more than the dose. An acid step first — white vinegar — dissolves the limescale that shields the biofilm and drops the pH; then rinse; then the percarbonate, which works best at pH 9.5–10.5. Do not mix the two. Vinegar plus bicarbonate in the same bowl neutralises into carbon dioxide and water, which is a satisfying fizz and zero cleaning power, and it is the single most common «I tried everything» that was never actually tried.
Three habits, fifteen seconds, worth more than any product
Dry the fold after every wash. One hand, one towel, ten seconds, and start at six o'clock because that is where the water is. This is the only single action that addresses the cause rather than the symptom. Leave the door ajar, not wide open — ajar gives you the half hour of ventilation the drum needs, while wide open leaves the gasket compressed against its own weight for years and deforms it; the detergent drawer wants to be open too. Once a month, run the hottest empty cycle you have — 90 °C tub clean, or 60 °C with a scoop of percarbonate — and cut liquid fabric softener right down, because it leaves an oily film on the rubber that is food.