Black tiger stripes down the front of gutters are oxidation and runoff staining bonded to the finish, not loose dirt sitting on top. A plain rinse will not remove them because the stain is in the surface, not on it.
What the stripes actually are
As water runs over the gutter face, it leaves a pattern of oxidation and organic staining that bonds to the painted aluminum. The result is the vertical “tiger stripe” pattern that a hose or light washing will not lift.
Why rinsing fails
Water alone does not break the bond between the oxidation and the finish. A gutter brightener — a specific chemical — dissolves the oxidation so it rinses away without damaging the paint.
Be gentle with the finish
Aggressive scrubbing or the wrong cleaner can dull or strip the gutter’s painted finish. The right brightener and a soft approach protect the coating while removing the stripes.
Five-minute self-check
- Run a wet finger down a stripe — if nothing lifts, it is bonded.
- Note whether the stripes follow the water runoff pattern.
- Check the finish for chalking or oxidation.
- Look at the downspout discharge points for matching staining.
- Decide whether a brightener is the right call.
Related reading
See our gutter cleaning page and our post on overflowing gutters.