How do you clean windows without leaving streaks?

Look at a pane that has just been washed and that marks. The streak appears as it dries, not during the washing. That single fact identifies the cause: it is not the original soiling that remains, it is a new deposit, manufactured by the wash itself.

Only three things can dry on a pane of glass:

• the minerals dissolved in the water — calcium, magnesium, silica — left behind when the water molecule evaporates
• the surfactant in the product, which nobody ever rinses off completely
• fibres and dust brought in by a cloth or by paper

The whole of the professional technique consists of making those three contributions disappear. It is not a matter of elbow grease: a pane scrubbed for ten minutes with hard water will dry exactly as badly as a pane scrubbed for two.

The water supplied in Marrakech is hard water. It is perfectly drinkable, and that is precisely the problem: what makes it drinkable — the mineral salts — is what marks the glass.

The trade measures this with a TDS meter, a conductivity meter that displays the quantity of dissolved solids in parts per million. On a mains tap in the city, the reading climbs into the several hundred ppm. Every one of those particles stays on the pane when the droplet evaporates. On a south-facing patio door, the water dries before it can even be wiped, and the deposit is immediate.

The professional answer is to remove those minerals before touching the glass. That is the principle of pure water: mains water passes through a sediment pre-filter, a reverse osmosis membrane, then an ion-exchange resin that captures the last of the salts. At the outlet we aim for under 10 ppm, checked on the TDS meter before every job.

Water at that level has nothing left to deposit: it can be left to air-dry on its own without leaving the slightest mark, which is exactly what happens when we work with a water-fed pole, between 3 and 5 m, where no wiping is possible at all.

A detail that surprises people: pure water is aggressive, in the chemical sense. Having lost its ions, it seeks to take some back and actively lifts the film of soiling. It is a solvent, not a passive rinse.

What separates a trained window cleaner from an amateur fits into one sentence: the squeegee does not leave the glass between the start and the end of the pane.

The common method — a horizontal band, lift, wipe the blade, put it back, start again — leaves a line at every re-entry. Those are the lines you find the next morning against the light.

The S-stroke removes the problem:

1. The strip washer is loaded, wrung out by hand, and passed over the whole surface to lift the film. That is the tool that cleans; the squeegee only clears.
2. The squeegee starts at the top left, runs diagonally down to the bottom of the pane, climbs back the other way, drops again — a continuous S that brings the water down.
3. The blade stays in permanent contact, angled at roughly 30 degrees, driven by the wrist and not by the arm.
4. A single pass along the bottom edge, with the blade or with a microfibre, closes the work.

Two things ruin everything else: a nicked rubber blade, which draws a line of water on every pass and must be reversed or replaced, and a squeegee width unsuited to the pane, which forces you to multiply the passes.

This is the most expensive mistake, and it cannot be undone.

The rubber of a squeegee is soft. A grain of sand picked up in the strip washer, or left sitting on the bottom of the frame, lodges under the blade, does not deform, and ends up pressed against the glass along the entire length of the stroke.

The result is a fine, clean, arc-shaped scratch, visible in raking light. No product, no paste, no polish available to you will remove it. The glazing is marked for life.

In Marrakech, where sand is everywhere and accumulates in the runners, that imposes a simple rule: dry-dust before wetting. Runners, sills, the bottom of the frame and the transoms are vacuumed or brushed dry first; the water comes afterwards. The reasoning is set out in full on cleaning sliding patio doors, where the runner concentrates most of the risk.

The corollary: a strip washer that has been used on a sandy window bottom does not go back up onto clean glazing without being rinsed. A bucket of dirty water is a liquid abrasive.

A pane has two faces, and a streak is visible from both. Against the light, the question is always the same: inside or outside?

The trade trick is to change the direction of the stroke deliberately depending on the face: vertically on the inside, horizontally on the outside. The day a mark appears, its orientation tells you immediately which side it is on, and you go back over one face only.

Two further points of method:

• Never wash in full sun. On a south-facing elevation in Marrakech, the glass can be burning hot by mid-morning; the water evaporates before the squeegee reaches it and the deposit is baked on. You follow the shade: east in the morning, west in the afternoon.
• The frame before the glass. An upright loaded with dust releases, at the first rain, a run that crosses the whole pane. That is why frames, runners and joinery are part of the service and are not charged separately.

The three remedies you find everywhere each carry a measurable defect.

• Washing-up liquid contains surfactants designed to stay attached to the surface. Two drops too many and the residual film attracts dust: the pane gets dirty again faster than before it was washed. In a professional strip washer, the dose is a few millilitres to a bucket, and it is there for glide, not for cleaning.
• White vinegar does genuinely dissolve limescale. It also attacks everything around it: a silicone seal repeatedly exposed to an acid loses its flexibility, crazes and eventually lifts at the foot of the glazing. On lacquered aluminium joinery, the same acid dulls the finish. It is a one-off fix, never a routine maintenance product.
• Newspaper worked on the inks of forty years ago. Today's inks migrate onto the glass and, worse, onto pale mastics, and the paper leaves fibres in the corners.

Add the most expensive case of all: abrasive products and scouring pads on coated glass — solar control, low-emissivity, self-cleaning. The functional coating sits on the surface. A single abrasive pass takes it off in patches, and the unit is fit only for replacement. On new glazing, the neighbouring subject — protective film left too long in the sun — is dealt with on post-construction window cleaning.

It has to be said plainly, because it is what an honest contractor establishes before taking the money: some panes will not come back clean.

1. Glass etched by limescale. A leaf sprayed for years by a drip line or by splashes from a pool ends up attacked in depth. The deposit is no longer on the glass, it is in the glass.
2. Misted double glazing. A veil sitting between the two panes signals a failure of the perimeter seal. Nobody reaches it without replacing the unit.
3. Scratched glass. Micro-scratches from a grain of sand, the arc left by a chipped blade: it is permanent.
4. Glass attacked by an acid during building work — cement remover, descaler badly rinsed. The mark is a surface etch, not soiling.

In those four cases we say so from the photos or at the visit, and we do not charge for a wash whose result is known in advance. It is the same logic of scope that governs all our window cleaning work in Marrakech: a limit announced beforehand is worth more than a disappointment afterwards.

For everything else — the overwhelming majority of cases — the question is simply whether the time you spend on it is worth the price: 75 MAD incl. VAT per window up to 2 m², whole frame and both sides included where reachable, with a 400 MAD minimum call-out charge.

Eight windows come to 600 MAD; four windows, 300 MAD, are raised to the 400 MAD minimum. The full grid is published on the window cleaning price list, and an estimate can be requested by describing your home here, with no commitment.