Water quality decides whether a skylight cleaning robot leaves spots, not the brush. Above roughly 150 ppm hardness, unfiltered water leaves mineral rings as it dries, and on sunlit glass those rings are the first thing anyone sees.
Skipping this is the most common quality complaint on skylight jobs. The robot cleans fine, then the agua evaporates and the panel looks worse than before. The fix is water treatment and drying, not more pressure.
How hard is your water, and why does it matter?
| Hardness (ppm) | Likely result | Action |
|---|---|---|
| Under 150 | Mostly spot-free when dried | Standard filtration |
| 150 – 300 | Faint mineral rings | Filter or blend with deionised water |
| Over 300 | Visible spots on sunlit glass | Deionised water or dry method |
You can test hardness with a cheap kit or ask the site for a water report. On regions with limestone geology, hardness often runs well over 300 ppm, and on those roofs filtered water is not optional.
Which filtration actually helps?
Two stages do most of the work: a sediment filter that catches particles and grit, and a deionisation or reverse-osmosis stage that removes dissolved minerals. The sediment filter protects the pump and the deionising stage protects the finish. Change the sediment filter weekly on dusty sites, and monitor the deionising resin by the water’s conductivity, not by a calendar.
Filter maintenance controls the result. A saturated resin stage passes minerals straight through while still looking clean, so the operator notices spots before noticing the cause.
Does drying technique beat water treatment?
Both matter, and they work together. Even perfect water leaves a film if it dries slowly in full sun, because airborne dust settles into the wet layer. Two habits help. Clean early morning while the glass is cool, and run a squeegee pass that lifts most of the water so drying time is short.
- Clean cool glass: early morning beats midday on any skylight.
- Two-stage squeegee: most of the water off, then a fast second pass.
- Avoid cleaning in the hour before rain, dust lands in the wet film.
- On visible atriums, a final dry buff removes the last haze.
When is the dry method the better choice?
On hard-water sites, high regions and interiors where any drip is a problem, minimum-water or dry cleaning with a soft brush leaves no minerals because it adds no liquid. The trade-off is a slightly hazier finish and more brush wear. For a factory sawtooth roof nobody inspects closely, dry wins on cost. For a sunlit atrium, wet with treated water wins on finish.
What about the glass coating under the water?
Self-cleaning and hydrophobic coatings change the answer. A coated panel repels water, so it dries in beads and spots form where the beads sit, which makes clean water even more important. Abrasive cleaning also wears the coating, and once it is gone the pane streaks for good. Check whether the roof has a coating before choosing a brush or a detergent.
Where coatings are present, use a soft brush, pH-neutral detergent between 6 and 8, and treated water. Skip acid-based descalers, which strip coatings faster than they remove minerals, and skip high-pressure rinsing on the glass itself. The coating is a thin film measured in microns; a season of rough cleaning removes it and no amount of polish brings it back.
How do you stop spots coming back after cleaning?
Spots return when the cause was never removed. If the water is hard, the minerals are in the supply, and no brush will fix it. If the glass is coated and the coating is failing, the streaks come from the coating, not the water. Diagnose before you re-clean, or you will keep repeating the same visit and blaming the machine.
A quick test: clean a one square metre test patch with distilled water and dry it fast. If it dries clear, your water supply is the problem and treatment is the answer. If it still spots, look at the coating or the drying method, because the water was clean and the marks came from somewhere else.
How much water does a spot-free job need?
Less than people think, if the water is treated. A minimum-water pass at around 0.03 to 0.05 litres per square metre with deionised water and a fast squeegee often beats a heavy rinse, because it leaves less water to evaporate and therefore fewer minerals to deposit. On a 400 m2 glass roof that is roughly 12 to 20 litres for the whole job.
This is why treatment pays for itself beyond the finish. Treated water used sparingly does the same job with less volume, less weight on the roof, fewer refills and a shorter drying window. The purchase cost of a small deionising unit is recovered in one season on a hard-water site.
What is the fastest way to diagnose spots?
Run the clean water test on the worst panel, then repeat it with a second pass at high squeegee speed. If the marks survive both, the minerals are bonding to the coating or embedded in a rough surface, and the panel may need a specialist treatment rather than another routine clean. Recognising that early saves a lot of repeat visits.
Keep a short log of which panels spot and which do not. Patterns emerge quickly, one facade may be fine while another spots every time, and the difference usually points to the water source, the sun exposure or the coating condition. The log turns a vague complaint into a fixable problem.
Key Takeaways
- Above 150 ppm hardness, treat the water or expect spots.
- Over 300 ppm, deionised water is effectively mandatory on visible glass.
- Sediment plus deionising stages cover most sites; maintain both.
- Clean cool glass early and squeegee fast to cut drying marks.
- Dry cleaning suits hard-water and no-drip sites at the cost of finish.
Getting spots on every job? Tell our team your water source through the contact page, or read the maintenance guide.