Phoenix Journal · Extraction
You can soak, scrub and re-soak and still hand a filter back that looks tired and grey. The reason usually is not effort - it is the kind of grease you are fighting and the metal it has soaked into.
Kitchen extraction
You have soaked them overnight, gone at them with a stiff brush and a good degreaser, and the filters still come out of the sink looking dull, sticky or streaked with brown - so what are you doing wrong?
Most likely, nothing. When a grease filter refuses to look clean no matter how hard you work at it, the problem is rarely your technique. It is usually one of two things: the grease has changed chemically into something a normal degreaser cannot lift, or the filter itself has been damaged by heat, harsh chemicals or age and simply cannot look new again. Knowing which of those you are dealing with saves a lot of wasted scrubbing - and tells you whether the filter needs a better clean or a replacement.
This page walks through why grease stops responding to cleaning, why the metal your filters are made from matters as much as the grease on them, and what genuinely gets them back to a safe, presentable state.
Grease straight off the griddle is soft and greasy, and it lifts easily with hot water and a degreaser. The trouble starts when that grease is heated again and again. Every service, the film on your filters is warmed, cooled and warmed once more by the rising heat from the range below. Over weeks, that repeated heating drives off the lighter oils and cross-links what is left into a hard, resinous varnish. Cooks often call it baked-on or burnt-on grease. The technical name is polymerised grease, and it behaves far more like a lacquer than like fat.
That is the crux of the problem. A degreaser is designed to break down oily, water-repelling grease and float it away. Polymerised grease has stopped being oily - it has set into a tough, slightly glassy coating that clings to the metal and shrugs off ordinary cleaning chemistry. You can soak it, scrub it and re-soak it, and it still leaves a brown, slightly tacky stain because the surface layer has genuinely bonded to the filter. This is also exactly why the extraction system behind the filter matters so much: any grease your filters miss travels into the canopy plenum, the ductwork and the fan, where the same baking process turns it into a hard fuel load. That build-up is the reason kitchen extract fires spread so quickly, and the reason the fire-safety cleaning standard, BESA's TR19® Grease, exists at all.
The other half of the answer is the filter itself. Commercial kitchens run two very different types, and they do not behave the same in the sink.
If you are not sure which you have, or you are weighing up a switch, it is worth reading our comparison of baffle filters against mesh filters before you spend money replacing like for like.
The quick answer
Before you write a filter off, it helps to know what you are actually up against. Three things decide whether a tired-looking filter is dirty, damaged or done.
That middle point catches a lot of kitchens out. If your filters keep coming out of the pot-wash a dull, patchy grey, the machine may be doing it. Commercial dishwasher detergents are strongly alkaline, and alkaline chemistry reacts with aluminium, oxidising the surface and turning it dark and chalky. The same happens when you leave aluminium mesh in a caustic soak too long. At that point the metal itself has been etched - the discolouration is baked into the surface and will not scrub off, because there is nothing left on top to remove. A stainless steel baffle filter, by contrast, will shrug off the same treatment and come back bright.
Once you know the enemy is polymerised grease rather than fresh fat, the method changes. Cold sprays and a quick wipe were never going to work. What does work is heat, time and the right chemistry, in that order.
It is also worth being honest about frequency. Filters that are cleaned little and often almost never polymerise in the first place, because the grease is removed while it is still soft. Filters left for weeks bake hard, and that is when the sink stops working. Most insurers write a weekly filter and canopy clean into the policy as a minimum, kept as a written record, and the periodic deep clean of the ductwork behind them runs on a heavier schedule - typically every three months for heavy frying and grilling, six months for a busy restaurant, and yearly for lighter use. If you are unsure where your kitchen sits, our guide to how often to clean a kitchen extraction system sets out the intervals in plain terms.
The filters, though, are only the visible tip. Every trace of grease they let slip is quietly polymerising inside the canopy, the ductwork and the extract fan · out of sight, harder to reach, and the part an insurer or fire officer will actually inspect. Filters you can rescue in a soak tank; a baked duct run needs specialist access and TR19® Grease cleaning to bare metal.
Questions
Because the grease has almost certainly polymerised. Repeated heating turns soft fat into a hard, varnish-like coating that has bonded to the metal, and ordinary degreasers are designed to lift oily grease rather than a set lacquer. A hot soak near 60°C over an hour or two softens it far better than cold soaking or scrubbing, but a filter left to bake for weeks may never look fully new again.
That grey is usually corrosion, not dirt, so no amount of scrubbing will shift it. Commercial dishwasher detergents and caustic degreasers are strongly alkaline, and alkaline chemistry reacts with aluminium and etches the surface. Use a gentler, aluminium-safe degreaser and keep soaks short - or switch to stainless steel baffle filters, which tolerate hot, strong cleaning without discolouring.
Look for damage rather than dirt. A filter that is warped, split at the seams, saturated right through the mesh, or etched a chalky grey by chemicals has reached the end of its life and will not seal or drain properly again. At that point replacement is cheaper than the labour wasted trying to rescue it, and it keeps your extraction working as intended.
Most insurers require filters and canopies to be cleaned at least weekly, with a written record kept for inspection, whatever your trading hours. Cleaning little and often is also the single best way to stop grease polymerising in the first place, because it is removed while still soft. The ductwork and fan behind the filters run on a heavier schedule, typically every three to twelve months depending on how hard the kitchen works.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.