With Ultramar Sail Cleaner, it is generally no problem to remove stains from spider droppings, flies and mosquitoes from a sail.
We always soak a sail with those kinds of stains in Ultramar Sail Cleaner for several days in our special sail laundry.
After 1 day we brush and rinse the sail, then soak it for another 24 hours. Generally that is sufficient. If necessary, you can repeat this treatment several times.