Hard water stains or scratches on glass: how to tell

How do I tell a hard water stain from a scratch?

They can look alike from across a room, but one sits on the glass and the other is cut into it. Three quick checks separate them.

• Wet the glass. A mineral stain almost disappears while wet and returns as it dries. A scratch looks the same wet or dry.
• Run a fingertip across it. Limescale feels rough, gritty and raised. A scratch feels like nothing, or like a fine groove.
• Use a fingernail. If the nail catches in the line, the scratch is deep.

Pattern and position help too. Stains follow water: spots in an arc where a sprinkler reaches, runs below a sill, a cloudy band across the lower half of a shower screen. Scratches follow movement: straight lines or arcs from a scraper, a ring, furniture, a pet or grit under a cloth.

There is a third condition between the two. When mineral deposits sit on glass for a long time in the sun, they attack the surface itself. This is etching: a haze that remains after the scale has been dissolved, smooth to the touch but permanently dull.

Why is glass in Dubai so prone to hard water stains?

The water is hard and the sun is fast. Mains and irrigation water carry dissolved minerals. When a sprinkler wets a window, the water evaporates in minutes and leaves the minerals behind, and the next cycle adds another layer. Within one summer the layers bond to the glass.

• Sprinklers aimed at or drifting onto windows, doors and balustrades.
• Shower screens that are left to dry by themselves.
• Glass washed with a hose in direct sun and not dried.
• Sea spray near the coast, which adds salt.
• Splashes around pools and water features.
• AC condensate dripping down a facade.

What can be polished out, and how?

Fresh mineral spots dissolve with a mild acidic descaler made for glass, followed by a thorough rinse. Built up scale needs a stronger specialist product and then machine polishing with a fine compound. The acid must be kept off marble, natural stone, aluminium frames and rubber seals, which it damages, so the surroundings are masked and rinsed.

Light scratches and mild etching are removed by polishing with cerium oxide on a felt pad. The process wears away a very thin layer of glass until the surface is level with the bottom of the scratch. That is why it only suits shallow damage. Polishing a deep scratch leaves a dip that bends the view like a lens.

Some glass cannot be polished on a given face. Coated, tinted and filmed surfaces lose their coating under the pad, so the type of glass is identified before any machine touches it.

When does glass have to be replaced?

• A scratch that catches a fingernail, in a position where the eye falls on it.
• Etching that still looks milky after a trial polish.
• Chips or cracks, especially at the edge of toughened glass, which can shatter without warning.
• Mist or water marks between the panes of a double glazed unit. The seal has failed and the inside cannot be cleaned.
• A damaged surface coating, or a laminated pane that is separating at the edge.
• Scratching across a whole pane, where polishing would take longer than fitting new glass.

Replacement glass must match the original in type, thickness and safety rating, particularly for doors, balustrades and shower screens. Building management may need to approve work on facade glass. Do not attempt to remove or carry large panes yourself.

How do I stop hard water marks coming back?

• Turn sprinkler heads away from glass and shorten their throw, or change them to drip lines near windows.
• Squeegee the shower screen after each use. It takes half a minute and prevents most build up.
• Wash glass in the shade or early in the day, and dry it before the sun does.
• Have external glass cleaned regularly with purified water, which dries without spots.
• Consider a water repellent coating on shower screens and exposed panes. It slows staining and needs renewing from time to time.
• Redirect any AC drain that drips onto glass.

Avoid the common mistakes: no scouring pads, steel wool or abrasive powders, no dry scraping with a blade, and no leaning out of windows or climbing to reach outside glass. The MELLS tests a small area first to show what polishing will achieve, and replaces glass only where that test fails. Book a site visit to have the panes assessed.