You may also try mixing one quart of water with 1 cup of bleach and 1 teaspoon of dish soap.
How to remove stains from white gutters.
If it is mold it will come of very easily with bleach.
Simple ready to use formula.
Try spraying an all purpose cleaner on the stains and scrub them off with a scrub brush.
It is actually pretty easy with the right material.
Rust residue dirt buildup and corrosion can all be removed by scrubbing the gutters clean.
Scrub the gutters with the soap mixture to clean them.
If you haven t cleaned the debris out of your gutter yet do this first with a garden trowel.
Dip a nylon scrubbing brush into the soapy water.
Brush the gutter with a stiff bristle brush.
Removes tough outside stains like mold mildew oxidation tree sap and bird droppings.
Apply the chemical according to the manufacturer s instructions.
Dip your scrub brush in this solution and scrub off the stains.
Wet the gutter with a garden hose after pre wetting the area.
Set up a ladder at one end of the gutter and fascia where the black streaks are located.
Your gutter will look as good as new.
Water based formula can also be used on mobile homes rv s and campers to remove black streaks and other stains.
Rinse with water and if necessary reapply the chemical to remaining stains and brush again.
Reposition the ladder when necessary to reach all parts of the gutter and fascia.
On stubborn streaks let the chemical dwell longer being careful not to let it dry.
I clean this for customers all the time and the product to use is called gutter zap.
Move down the entire gutter and spray all of the spots with black streaks.
Climb up the ladder and spray the black streaks with gutter cleaner.
I assume from what you have described it is a white gutter with aluminum oxidation on it.
Using a power washer clean your gutters to get the initial mildew and grime off.
This is a product that will remove them with remarkable ease.
These may require you to vigorously scrub or make take several attempts but it can all be removed.