Quick answer: Clean the outside first (it's always dirtier — do it second and you'll chase "streaks" that are actually outside grime), then the inside. Use the pro's direction trick: horizontal strokes on the inside, vertical on the outside — any streak instantly tells you which face it's on, ending the wipe-the-wrong-side-five-times ritual. Different dirt, different chemistry: outside wants a wash (dust, spotting, film), inside wants a degreasing mist (hand oils, cooking film, condensation residue). The genuine speed unlock: run a robot on one face while you do the other.
Everyone knows the dance: you clean a window, spot a streak, wipe it, it's still there, wipe harder, still there — because it was on the other side all along. Two-side cleaning has exactly two secrets, and they're both about information, not effort.
Secret 1: outside first, always
The outside face carries 80% of the dirt — weather spotting, dust, pollen, traffic film — and until it's clean, you cannot judge the inside at all. Clean inside-first and every remaining outside blemish reads as your streak. Outside, then inside, then judge.
Secret 2: cross your strokes
Finish the outside face with vertical strokes, the inside with horizontal ones (or your own convention — just make them different). Now every streak carries its own address: horizontal streak, inside face; vertical, outside. This one habit, standard among professionals, saves more time than any gadget under fifty quid.
The outside face: it wants a wash
Outside dirt is particulate and mineral. Wash with 1:4 vinegar solution and a microfibre or washer sleeve, squeegee off in overlapping vertical lanes, dry the bottom edge and cill. Gritty glass gets a rinse before anything touches it. Mineral spotting gets its acid dwell time. Upstairs outsides — the faces that make people abandon the whole idea — are the robot's signature move: place from the open window, rope anchored, done without a ladder.
The inside face: it wants a degrease
Inside film is organic — hand oils, cooking vapour, candle soot, the residue that condensation leaves as it dries. It needs less liquid and more grab: a light mist of vinegar solution (with that single drop of washing-up liquid for kitchens), tight horizontal strokes, dry microfibre finish. Less is more indoors — oversprayed cleaner running onto cills and paint causes more mess than it removes.
The halving trick: parallel processing
The actual fastest way to do both sides: don't do them in sequence. Put a CleanBot R1 on the outside face and work the inside by hand at the same window — the robot maps and covers its face in overlapping passes while you do the two-minute inside wipe, and you meet in the middle with a fully finished window in the time one face used to take. Then the robot moves to the next pane while you follow. A whole house's both-sides clean stops being a weekend project and becomes an accompanied hour, and the strokes trick still works: the robot's pattern is distinct from your hand strokes, so attribution stays instant. (The worth-it maths, if that hour convinced you.)
FAQ
Best weather for the two-side clean?
Overcast and mild. Direct sun flash-dries solution mid-wipe — the classic cause of sudden inexplicable streaking.
How often for each face?
Outside: quarterly baseline, more by postcode. Inside: monthly in living areas, weekly for kitchen and bathroom glass.
Newspaper — genius or myth?
Legacy technique from when ink and glass were different: modern inks smear and today's microfibre outperforms it comprehensively. Let it retire.
Outside first, crossed strokes, right chemistry per face — and a robot doing half the window while you do the other. Both sides, no mysteries, half the time.