I would be VERY careful. On the issue of mildew; the best you can do is use a mixture of bleach but don't use more than 2% bleach by volume with water. Let the sail soak with it and keep it wet. You could use a bathtub or a large clean plastic garbage can if you want to soak it overnight.
To wash your sail lay it out on a clean lawn and mix woolite and a scoop of Oxyclean. Don't use bleach if the sail looks good with little mildew. Scrub it with a soft bristle brush (car washing pole brush); rinse well and then allow it to dry in the sun. Just turn it every hour or so and it will dry out despite sitting on the lawn.