The commercial banana plantations spray their banana bunches with a copper sulfate mix ( Bordeaux mix)
to prevent several types of fungus infections while the fingers are maturing. This includes the types that start as spots. ... Also, after harvest the bunches are rinsed/washed in the bleach water to prevent any developing infections and lengthen shelf life of the banana. This is how the plantations keep the banana skin clear and un-stained.
I do spray my bunches with the copper mix after the male bud develops before any infections starts and spray the hanging bunches after harvest with bleach water to drip dry. ... The spots can not be removed if already started, but the infection can be stopped before getting to the fruit if treated quickly.