Scotchguard® and related water repellents do not have any properties that either kill or prevent the recurrence of mildew. They may be marginally effective at repelling some of the moisture and nutrients on which mildew
feeds, and might make cleaning easier by holding the stains away from the fibres. There is not much evidence either way on this.
Dettol®, a commonly available household disinfectant, is the most powerful and effective fungicide and inhibitor you can use to prevent recurrence and spread of mildew. Various health and environmental agencies prohibit the
use of stronger fungicides since the same thing that kills fungus has similar effects on higher life forms, as most of us would like to picture ourselves.
Anything you use on a sail to kill or remove mildew and stains, will wash or wear away in a relatively short time. This is directly analogous to Yacht anti-fouling bottom paints. Although not used on Contender sails, North NorLamTM fabrics are treated with the most powerful commercial fungicide we can use without jeopardizing the health of employees and customers. It
is100% effective in preventing mildew in laboratory conditions, and demonstrably less effective in the real world.