I personally use the old formula of Scotchguard in the GREEN can. It works, I cannot speak for the new RED can formula.
Others have said that the the silicone waterproofing for tents works.
I spray the Scotchguard into a glass jar with a cap like the gourmet mustard or fruit preserves you get at high end restaurants. They are just the right size. Dip fly in, shake, and allow to dry.
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy