Spot Pattern Tells Cause: Droplet vs Streak vs Film Single-Observable Decision Tree

Why this matters

The shape of what is left on the glass is a fingerprint of how it got there. A senior cleaner does not need to re-clean a window to know why it failed; the pattern alone narrows the cause to one or two mechanisms. Discrete round droplets, linear streaks, and uniform film are produced by different physical processes, and confusing them sends you chasing the wrong fix. Reading the single observable pattern first, before touching anything, lets you arrive at a callback already knowing whether it is your water, your rubber, your rinse, or the glass itself. This tree maps each pattern to its mechanism and the confirming test.

Symptom presentation

Look at the dry glass in raking light from the side. Three mutually exclusive baseline patterns:

• Droplets: discrete round or run-down marks, often with a mineral ring, located where water beaded and dried in place.
• Streaks: linear marks, usually parallel, following the direction of a squeegee pass or a brush stroke.
• Film: a uniform haze across the whole pane with no discrete features, visible mainly as reduced clarity in light.

A pane can show a combination, but identify the dominant pattern first and treat secondary patterns as a second pass.

Quick checks

For each dominant pattern, one quick check confirms the family before you diagnose deeper:

• Droplets: do they wipe off dry. If yes, fresh mineral deposit. If no, etch.
• Streaks: are they straight and directional (squeegee/brush) or random (residue smear). Directional points to tooling; random points to product residue.
• Film: does it wipe to clear with a dry cloth (residue) or stay (etch or interpane).

Isolation tree

Droplet branch: Round marks mean water beaded, sat, and dried with its dissolved solids left behind. On a WFP job this is rinse run-off or impure water; read brush-head TDS and confirm the final flush did not leave standing beads at the bottom rail. On a traditional job it is splash-back or detailing water that was not removed. If droplets do not wipe off, they are pre-existing hard-water etch the clean exposed, concentrated low on the glass near sprinkler throw or under run-off lines. Droplet pattern plus a sprinkler-adjacent location is the signature of irrigation overspray, not your service.

Streak branch: Directional parallel streaks following squeegee passes mean the rubber is worn, nicked, or the channel was dirty, or the detergent film was too thin to lubricate the pass. Streaks following brush strokes on a WFP job mean the brush lifted off while too wet or the final rinse did not flush across the full pane. Random non-directional streaks that smear are product residue: too much detergent, a dirty cloth, or a glass cleaner with additives that flash-dry. Confirm by the direction test; tooling streaks line up, residue streaks do not.

Film branch: Uniform haze that wipes to clear is residue: hard-water minerals in the wash water, detergent left un-rinsed, or a film-forming additive in the cleaning solution. Uniform haze that does not wipe is either true etch (acid test required) or interpane contamination from a failed insulated-glass seal (cannot be cleaned). On WFP, a faint uniform film at good TDS usually means the glass was rinsed but the frame water sheeted over the glass during run-off, depositing frame minerals.

Confirming diagnosis

• Droplet, wipes off: brush-head TDS reading. Above 010 ppm confirms water; at 000 to 010 ppm confirms rinse run-off technique.
• Droplet, does not wipe, low and sprinkler-adjacent: this is pre-existing irrigation etch; an oxalic/sulfamic test patch will lighten a removable deposit and not move a true etch.
• Streak, directional: inspect the squeegee rubber edge under a loupe for nicks; replace and re-do one pane to confirm.
• Streak, random smear: re-clean with clean water and a fresh cloth, no additive; if it clears, it was residue.
• Film, wipes clear: residue confirmed; trace to wash-water hardness or over-dosed detergent.
• Film, stays: acid test patch to separate etch from interpane; if neither moves it, suspect IG seal failure and inspect the spacer for fogging.

Why pattern is the fast path

Reading the pattern first short-circuits a lot of re-cleaning. Each pattern is the physical record of a different failure: droplets record water that beaded and dried in place, streaks record a tool or cloth dragging across the glass, and film records a uniform layer that either deposited from solution or was always there. Because the three are produced by mutually exclusive mechanisms, identifying the dominant pattern in raking light eliminates two-thirds of the suspect list before you take a single reading. The only readings you then need are the confirming ones for the branch the pattern already pointed you to: a TDS check for droplets, a rubber inspection for directional streaks, an acid patch for non-wiping film. On a WFP job, the combination of a droplet pattern at the bottom rail plus a good brush-head TDS is the classic signature of standing rinse water that was not flushed off the bottom edge, a pure technique fix; the same droplet pattern with a high brush TDS is purely a water fix. The pattern plus one number resolves it.

Remediation

Droplet/water: service the purification stage, re-rinse. Droplet/etch: disclose as pre-existing, quote restoration separately. Directional streak: replace rubber, clean the channel, ensure adequate detergent film, or retrain brush lift-off and final flush on WFP. Residue film or smear: switch to pure or softened wash water, reduce detergent, use clean microfiber, drop film-forming additives. Etch or IG film: document and refer; no cleaning method removes it.

References

• International Window Cleaning Association (IWCA), I-14.1 Window Cleaning Safety Standard.
• ASTM C1036, Standard Specification for Flat Glass (surface-quality reference for distinguishing deposit, etch, and inherent defect).
• US EPA, Secondary Drinking Water Standards (TDS and hardness guidance relevant to wash- and rinse-water quality).
• IWCA technical guidance on pure-water systems and spot-free rinse technique.