Check whether the mark is on the surface

Clean a small area gently with the mirror maker’s approved method and a soft cloth. If the dark edge stays behind a clean glass face, polishing harder is unlikely to help. Do not scrape it with a blade or soak the edge to test your theory. Photograph the marks in even light so you can see whether they are spreading.

Kohler identifies dark edge spots and streaks as signs of desilvering, with moisture a main cause. Its guidance also warns that ammonia- and vinegar-based cleaners can accelerate the problem. Kohler: Troubleshooting desilvering.

Surface film versus backing damage

  • Film on the face

    May change after a gentle, compatible cleaning test.

  • Darkness behind clean glass

    Points toward deterioration of the reflective backing.

  • Growing edge marks

    Document the pattern and address water reaching the edge.

Change where the cleaner goes

Spray the cloth rather than the mirror. Wipe without flooding the bottom edge, then dry any collected droplets. A mirror mounted just above the backsplash can catch cleaner and splashes in a narrow gap that stays wet long after the face looks dry. Keep that gap clean without pushing liquid into it.

Use the fan during and after bathing according to the ventilation plan, and deal with persistent condensation. A mirror does not need to become completely fog-free the instant a shower starts, but water repeatedly sitting along its lower edge is worth addressing. Check for a dripping light, plumbing leak or splash source if damage is unusually local.

Repair the mirror or replace the part?

A glass specialist can advise on resilvering a valuable mirror. For an ordinary bathroom mirror, compare that quote with safe removal and replacement. On a medicine cabinet, the manufacturer may offer a replacement door, which can preserve the cabinet body and avoid tile disturbance. Confirm hinge compatibility and glass dimensions before ordering.

Adding a frame can conceal a narrow damaged border, but it does not restore the backing or remove the moisture source. Do not trap a wet edge behind new trim. If replacement also means changing storage, review the recessed-cabinet checks before cutting the wall.