Pressure Washing
If your white gutters have dark vertical streaks running down the face, you have what the trade calls tiger striping. It looks like dirt washing down from the roof, but it's actually a bonded film — a mix of airborne pollution, oxidized aluminum, and roof runoff residue that etches into the gutter's painted finish.
Tiger stripes are not loose surface dirt, so spraying them with a garden hose or even a pressure washer barely fades them. The grime has chemically bonded to the coating and has to be broken down, not blasted off.
Pros use a dedicated gutter cleaner — a mildly acidic or citric detergent — applied with a soft brush or low pressure, allowed to dwell, then rinsed. The surfactants dissolve the bonded film and restore the original white. It's hand work, not a quick blast, which is why results depend on the product and technique.
Clean gutter faces at the same time you soft-wash the house so the film never fully sets, and clear the insides so overflowing debris doesn't restreak them. Many homeowners bundle it with a full exterior wash — the Capital City pros we suggest around Tallahassee treat gutter brightening as part of a house wash rather than an add-on afterthought. If you only remember your gutters after they're already striped, expect the first cleaning to take real effort.