Quick answer: Lightly mist the pad or the glass with a 1:4 white vinegar and water mix, or use purified water alone — that's it. The pad should be damp, never wet: a soaked pad smears, drips down the pane and can weaken the suction seal at the edges. Never use washing-up liquid or foaming glass cleaners in or under a robot (residue film + foam under the seal), and never ammonia products on tinted or filmed glass. Cost of the right solution: pennies per month.
The pad-and-suction design of a robot window cleaner does the mechanical work — the solution's only job is to loosen the film so the microfibre can lift it. Get the solution wrong and you get the classic complaints: streaks, hazy arcs, or a robot that seems to "clean worse than my hand does." It isn't the robot; it's the chemistry.
The golden rule: damp, not wet
A robot pad should feel like a wrung-out cloth — damp to the touch with nothing to drip. Two or three mists of solution across the pad is enough. Oversaturate and three things go wrong: liquid smears instead of lifting, drips run down and dry into streaks below the cleaned area, and moisture creeping under the suction skirt costs grip. If you can squeeze a drop out of the pad, it's too wet.
What to use
Option 1 — vinegar mix (the everyday choice): one part white vinegar to four parts water in a spray bottle. Vinegar dissolves mineral film and light grease and evaporates to nothing, which is the entire secret of streak-free glass. Total cost: about 20p a litre.
Option 2 — purified water (the perfectionist's choice): deionised or distilled water leaves zero residue by definition. It's what water-fed-pole professionals use. Best for maintenance cleans on already-decent glass, and the only thing to use on solar panels.
Option 3 — the DIY streak-free mix (for grubbier glass): in 500ml of water: 100ml white vinegar, a teaspoon of isopropyl alcohol (faster drying), and a single drop — genuinely one — of washing-up liquid to break surface tension. More soap than that and you've recreated the residue problem that causes streaks.
What never to use
- Washing-up liquid at normal dilution — leaves a sticky film that re-attracts dirt within days, and foam under the seal is bad news.
- Foaming aerosol glass cleaners — same problems, plus propellant residue.
- Ammonia-based cleaners — attack window films, some tints and rubber seals.
- Abrasive or cream cleaners — they belong nowhere near microfibre or coated glass.
- Anything on a dry pad against gritty glass — rinse visibly gritty glass first; grit under a pad scratches, whoever's driving.
Match the solution to the job
Everyday film: vinegar mix. Hard-water spotting: stronger vinegar pass by hand first, then robot maintenance. Traffic film: one manual degrease, then the robot holds the line. The pattern: the robot is the maintenance system; deep restoration is a one-off manual job before automation takes over.
FAQ
Spray the glass or the pad?
The pad, usually — even coverage, no drips. Misting the glass works on large panes in warm weather when the pad dries mid-clean.
Can I run the robot with a completely dry pad?
Yes — a dry pass is actually the right finish for polish, and ideal for dusty-but-not-grimy internal glass and mirrors.
How often should pads be washed?
Every few cleans, and machine-washed without fabric softener — softener coats the fibres and kills their grab. Full pad care guide here, and fresh R1 pads are £27 for six.
Right solution, clean pads, and the CleanBot R1 will out-clean a Sunday afternoon of your best squeegee work — every week, without you.