Hard water spots are calcium and magnesium carbonate left when water evaporates on glass. Above 150 mg/L hardness they build fast on hot skylights. The cure is prevention: deionised water, squeegee recovery and cleaning before the pane heats up. Once etched, only a cerium-oxide polish removes them.
Why do skylights spot more than windows?
Angle and heat. A vertical facade lets water run off. A pitched skylight holds droplets in the field of the pane, and the glass runs hotter, so evaporation is quicker and minerals concentrate in rings. Runoff from a dirty frame or a leaking gutter adds silica and rust, which bind harder than carbonate alone.
- Under 120 mg/L hardness: light spotting, mostly cosmetic.
- 120-200 mg/L: visible rings within one dry cycle on hot glass.
- Over 200 mg/L: etching risk; deionised water is mandatory, not optional.
How do I stop spots before they form?
Three levers. Reduce dissolved solids by using deionised or RO water in the tank. Recover the rinse water with a squeegee so nothing dries in place. And clean when the glass is cool, because warm glass dries droplets faster than the squeegee can chase them. Lingyun Y3’s 20 L tank plus a filter cartridge keeps flow low and controlled, which matters more here than raw speed.
The mistake people make is adding more water to rinse off spots. Tap water adds more minerals. A single wet pass, squeegee, done, beats three rinses with hard water every time.
Can a robot remove old etched spots?
No. A cleaning robot handles dirt, dust, pollen and light film. It will not cut through etched mineral scale, and running one repeatedly over etched glass just wears cups and wastes water. Etched panes need a dedicated polish with cerium oxide or a professional glass resurfacing step first. Confirm the glazing supplier approves polishing – coated low-E panes can be ruined by abrasives.
Is the water supply the real problem?
Often, yes. If the building’s tap water tests over 200 mg/L, no robot setting will save you. Budget a small RO unit or buy deionised water. It costs money per litre, but far less than replacing clouded atrium glazing. Track water hardness the same way you track filter changes.
Who can ignore this?
Buildings in soft-water regions with cool, steep glass and vertical facades. If your water is under 120 mg/L and your skylights drain freely, ordinary cleaning plus a squeegee is enough. Hard-water sites, hot climates and shallow-pitch roofs need the full regime.
How do you test and treat the water?
A cheap hardness test strip tells you enough. If the reading is high, add a small RO or deionising unit or buy cartridges. Change filter media on a schedule, not on feel, because a saturated cartridge passes minerals straight through. Log hardness each quarter alongside filter changes so you can see when the supply shifts.
What about rain and pollen between cleans?
Rain helps and hurts. It rinses loose dust but carries minerals from the roof and gutters, so the frame edge spots more. Pollen season adds a sticky film that binds dust. Adding one clean after the pollen peak, or a light rinse of frames, keeps the glass from building a hard layer that later needs polishing.
Can spotting be a coating problem rather than water?
Sometimes. A failing hydrophobic coating can leave patchy water marks even with soft water. If hardness is low, spotting is still heavy, and the pattern follows the coating, suspect the glazing rather than the cleaning. Coating damage is not reversible with cleaning; note it and raise it with the glazing supplier before replacing cups and filters chasing the wrong cause.
How often should filters be changed?
Follow the maker’s interval and shorten it on hard-water sites. A safe habit is to check flow and pressure at every clean and replace media at the stated interval or sooner. A clogged filter slows output and can push operators to open the flow, which defeats the whole low-water approach. Change on schedule and log it.
Does the squeegee add much cost?
Little, and it saves a lot. A good squeegee and recovered-water flow cost more upfront than a simple spray-and-go setup, but they cut the volume of water that dries on the glass. On hard-water sites that single difference is the gap between clean glass and a fresh set of rings. If the budget is tight, spend it on recovery rather than on speed.
Can you polish with the robot?
Not for etched scale, no. Polishing needs controlled pressure and a cerium compound applied by hand or with a dedicated machine, and it changes the glass surface. Keep the two jobs separate: clean with the robot, polish the specific damaged panes by hand. Mixing them risks wearing the robot’s cups and leaving an uneven finish.
Keep a simple log of hardness, filter changes and spot appearance. Over a year it shows whether the water is stable and whether a treatment change actually helped. Without the record, spotting complaints become a guessing game between the water supplier, the glazing and the cleaning team, and nobody wins that argument.
Finally, ask your supplier what water the machine was tested with. A robot validated on soft water may behave differently in your hard-water region, and the honest answer tells you whether the maker understands spotting or just ships boxes.
Key Takeaways
- Spots are mineral residue; heat and shallow pitch make skylights worst.
- Above 200 mg/L hardness, deionised or RO water is mandatory.
- Squeegee recovery and cool-glass timing prevent most spotting.
- Robots remove dirt, not etched scale; polish etched panes separately.
- Test water hardness before blaming the machine or the operator.
Ask about filtration options for your site via the contact page. Water-handling notes are collected in the news archive.