Re: Mildew stains
The manufacturer of the vinyl in my boat recommends for mildew stains a mixture of one tablespoon of ammonia, 1/4 cup of hydrogen peroxide, and 3/4 cup of water. Rinse several times with clear water.
Do not know if this works on canvas or other materials...like all instructions say...try product in an inconspicuous spot to test for compatibility...in other words, try at own risk.