· by Michael Blair, Nashville's Pressure Washing
Tiger striping is the pattern of dark vertical streaks that forms on the outside face of aluminum gutters, heaviest where water drips over the front edge. It is not mildew and it is not ordinary dirt, which is why the house wash that clears the siding leaves it behind. Understanding what it is made of explains why the usual approaches fail.
What the stripes are
Gutters are painted aluminum, and the factory finish slowly oxidizes at the surface into a fine chalky layer. Rain and dew running over the front lip pick up that chalk together with airborne pollutants, road film and the tar-like residue that washes off asphalt shingles, and deposit the mixture in streaks as the water evaporates on the face. The result is electrostatically bonded to the oxidized paint, not sitting on top of it.
The streaks are darkest directly below any point where water drips over the edge, which is why they line up with the shingle tabs above and why they are worst on the shaded sides of the house where the face stays damp longer.
Why pressure and house wash do not work
Water pressure removes what is loose. The stripes are bonded to the paint, so pressure washing takes off the surrounding chalk and leaves the streaks in higher contrast than before. Turn the pressure up enough to shift them and the finish comes with them, leaving bare aluminum that oxidizes far faster and stripes worse the following year.
The sodium hypochlorite mix used to soft wash siding kills mildew and lifts organic grime. The stripes are largely inorganic, oxidized paint and hydrocarbon film, and bleach does little to them beyond lightening the surrounding surface.
What removes them
A gutter-specific cleaner, sold as gutter whitener or gutter brightener, applied to a dry or lightly wetted surface and agitated with a soft brush or pad. These are alkaline degreasers, sometimes with a mild abrasive, formulated to release the bonded film without cutting into the finish. They are applied section by section, worked in, and rinsed with low pressure before they dry.
On heavy striping, a second application is common. On gutters where the paint has already oxidized to the point of chalking heavily, the cleaner will remove the stripes and reveal a faded finish underneath, which is the honest condition of the gutter and not something a cleaner can fix.
- Alkaline gutter cleaner, not bleach and not pressure
- Soft brush or white pad, worked along the length
- Low-pressure rinse before it dries
- Protect plants and painted surfaces below; the cleaners are strong
Keeping them off
The striping returns because the oxidation and the drip pattern do not change. Doing the gutter faces as part of each house wash, before the buildup bonds hard, keeps it to a light job. An applied polymer or wax sealant over clean gutters slows re-soiling and is worth doing on the shaded elevations where it is worst.
Correcting the drip helps too. Gutters that overflow at the front edge stripe far faster, so pitch, clean troughs and a properly installed drip edge that sends water into the gutter rather than over its lip all reduce it. On a gutter that has been striped for years and chalks heavily, a repaint with a coating made for aluminum is the point at which cleaning stops being the answer.