Finding faint lines or scuff marks after your robot runs is almost always a surface or brush issue, not permanent damage — and it is usually easy to stop.
Grit Trapped in Brushes or Tracks
The most common cause is fine sand or grit lodged in the brushes or tracks, which the robot then drags across the surface like fine sandpaper. Flip the robot over and rinse the brushes, tracks, and impeller thoroughly after sandy or dirty cycles. This is closely related to the concern about a robot damaging a pool liner.
Worn or Hardened Brushes
Brushes that have hardened or worn smooth lose their gentle scrubbing action and can leave streaks. Swapping in fresh, soft brushes restores clean scrubbing and protects the surface — find them on our parts page. Soft foam or rubber brushes are especially important on vinyl pools.
It May Be Algae, Not Marks
Often the "lines" are actually a thin algae or biofilm layer being smeared rather than removed, which shows up as the robot's path. In that case the fix is water chemistry plus fine filtration: balance and brush the pool, and run the robot with NanoFilters to actually capture the film instead of pushing it around. More on this in do pool robots pick up algae. A strong scrubber like the Dolphin Sigma lifts biofilm cleanly instead of streaking it.
