None so Blind--- The problem of mildew on fly blinds

fredmil

Our caravan spends the winter parked at the side of the house.  The fly blinds collect small black marks that are difficult to remove. These appear to be mildew and arrive every spring . It does not make any difference whether the blinds are raised or lowered during the winter.  My service engineer recommends a weak solution of Milton and water applied with a soft toothbrush. This works but is a very slow process. Any ideas on how to avoid the problem would be appreciated. Club technical notes on care of blinds do not deal with this.