Run a finger along the outside sill of any flat on a main road and it comes back black. Properly black, and greasy with it, the kind of mark that gets under a fingernail and stays there. That’s London traffic doing what it does to the outside of a building, one exhaust at a time.
People assume it’ll wipe off like ordinary dust. It won’t, and the reason why decides how you tackle it.
Why does traffic grime cling to window sills the way it does?
Traffic film is a thin, oily skin of diesel soot and brake dust, bound together with the greasy residue a busy road throws off every dry day. The soot gives it the black colour and the grit comes from brake and tyre dust, while the oil is what makes it stick like it’s been painted on.
That oil is the whole problem. It bonds the black film to whatever the sill’s made of and shrugs off water, because oil and water want nothing to do with each other. Wipe a grimy sill with a wet cloth and you smear the film into a grey haze rather than lifting it, and you’ll swear the sill’s clean until it dries and the shadow comes back.
South and west-facing sills bake it on harder. Sun warms the oily film through the afternoon and bakes it onto the surface, so the sunny side of a building is usually the stubborn side to clean.
Why won’t plain water shift it?
Same reason washing a greasy plate under a cold tap gets you nowhere. You need something that breaks the oil – a degreaser – before water can carry the dirt away. Plain water can’t grab an oily film, so it rides over the top and moves it about. Warm water helps, a squirt of washing-up liquid helps more, and a proper degreaser turns a two-hour scrub into a ten-minute wipe. That’s half the battle; most people are just reaching for the wrong weapon.
What’s the right way to clean uPVC frames without wrecking them?
Start dry. Brush or vacuum the loose grit out of the frame and off the sill before a drop of water goes near it, because wet grit is a grinding paste, and dragging it across uPVC leaves fine scratches that trap the next lot of dirt and never come clean again. Get the dry grit off first and you’re half the battle done.
Then the wet work. Warm water with washing-up liquid does an ordinary sill. For a badly grimed main-road frame, my secret is car shampoo or traffic-film remover – the stuff sold for cleaning the lower panels of a van. It’s built for exactly this grime, costs a few quid a bottle, and walks all over the “uPVC cleaner” sprays that charge three times as much for a weaker mix. Wet it and give it a minute to bite, then wipe it off and rinse.
Work in sections and rinse each one before it dries, because a degreaser left to dry back on uPVC leaves its own faint film you’ll then be chasing. A microfibre cloth for the final wipe grabs the last of the loosened grime instead of pushing it round, and it won’t scratch. Two careful passes on a bad frame beat scrubbing one spot to death.
What you keep well away from uPVC is anything abrasive – no cream cleaner, no scouring pad. Scratch the smooth surface once and you’ve given the grime a key to hold onto for good.
Can you get yellowed uPVC white again?
Mostly, no. A yellowed or chalky frame hasn’t got dirt on it at all – the sun has broken down the plastic itself at the surface, and no cleaner reaches under that because there’s nothing on top to remove. The restorer creams and uPVC “revivers” put a wet-look film over the top that flatters it for a few weeks and then washes off, leaving you back where you started. If a frame’s gone properly yellow and blotchy, that’s the frame ageing, and cleaning was never going to fix it. Save your money for the frames that are only dirty.
How do you clean painted timber and stone sills safely?
Old painted timber wants a lighter hand than uPVC, because the grime is often the least of what’s going on. Traffic film loves to sit on old gloss, and where it holds damp against the paint you get flaking and, on a shaded sill, a spread of black mould in among the soot. Sugar soap and a soft cloth lift the film without stripping sound paint, but work gently around any edge that’s already lifting or you’ll take good paint off with the dirt. Where there’s mould as well as grime, a dilute mould wash does what a degreaser can’t, and the two jobs are separate ones.
Do the mould wash and let it sit its full time before you rinse, or you kill the surface growth and leave the roots to come straight back through by autumn. On a north-facing timber sill in a shaded side return, that green-black film is half the reason the paint’s failing underneath, so clearing it earns its keep beyond the look of the thing.
Stone and concrete sills are their own thing. The soft trap here is reaching for something acidic to cut the black – on limestone or a lot of London’s older stone, acid etches the surface and leaves a dull patch that gathers dirt faster than before. Warm water and a stiff brush, a pH-neutral cleaner if you need more, and patience. That’s the lot. If the stone’s been painted at some point, treat it as paint, not stone – masonry paint on a sill holds grime differently and cleans more like the timber does.
What’s the black crust on an old stone sill?
Sometimes what looks like grime on old stone has become part of the stone by now. On Victorian and older buildings the black crusting you see in sheltered spots is sulphation – decades of sooty, sulphurous air reacting with the stone to form a hard gypsum crust with the pollution baked into it. It’s a conservation matter, and scrubbing at it hard or blasting it does more harm than the dirty look ever did. On a listed or period frontage, that black is a job for a proper stone cleaner rather than a bucket and a brush. Leave it be and clean the sound stone around it.
What about the seals, corners and drainage holes?
This is the bit everyone forgets, and it’s half the battle on a frame that keeps looking dirty however hard you clean the flat faces.
Grime collects worst in the corners of the frame and along the rubber gasket where the frame meets the glass, and an old toothbrush dipped in your degreaser gets in where a cloth can’t. Run it round the bead and into each corner and you’ll be surprised how much black comes out of a frame you thought was clean.
Go easy on the gasket itself. That rubber seal perishes with age and UV, and if it’s already hard and cracked, a toothbrush and degreaser will clean the grime out of it but won’t bring it back – a perished seal is a replacement job, and the black you’re scrubbing at is sometimes the rubber breaking down rather than dirt at all.
The bit that matters most, though, sits along the bottom of a uPVC frame: the drainage slots, the little slotted holes that let rainwater out. Traffic grime and moss block them solid, and a blocked one means water backs up inside the frame and finds its way indoors instead of out. Poke them clear with a matchstick or a cable tie every time you clean. A tenant in a mansion block on Marylebone Road had me out for a “leaking window” that was nothing but three weep holes packed with black London gunk – cleared in a minute, and the leak with them.
Should you ever take a pressure washer to a window?
No. I’ll be blunt about that one. A pressure washer feels like the obvious tool for baked-on grime, and it’ll drive water straight past the seals and gaskets and into the wall and the frame, and you trade a dirty sill for a wet plaster patch under the window and a frame that mists up for a fortnight. The force strips the fine detail off old paint and stone as well. A brush and a bucket of degreaser, with a garden hose on a gentle flow for rinsing – that’s as much water pressure as a window ever wants.
How often should main-road windows be done, and how do you keep on top of it?
More often than you’d think, and it depends entirely on the road. A window over a bus route or a main drag in Shepherd’s Bush takes on more grime in three months than a back-street window gathers in two years, and there’s no shame in the difference – it’s what living on a main road costs in cleaning. For a busy frontage, twice a year keeps it from ever building into the baked-on state that needs the hard work. A quieter street, once a year with the spring clean is plenty.
Does which way the window faces matter?
It does, on top of the traffic. The sunny sills bake the film on hardest, so they need the degreaser and the patience. North-facing sills get less sun but hold damp longer, so those are where the black mould and green algae set up alongside the soot, and they want the mould wash as much as the degreaser. A ground-floor window on the road takes the worst of both, splashback off the wet street included, and earns a clean more often than the flats above it. Once you know which of your windows is the bad one, you can spend your effort there and give the sheltered ones a quick going-over.
Keep an old toothbrush and a bottle of car shampoo under the sink, and the job never gets big enough to dread.