Sail Preserver?

In Sail Magazines "Tip of the Week" (http://www.sailmagazine.com/weekly-tip) I see:

Sail Saver
If you have older Dacron sails, a good method of preserving them while improving their working ability is to treat them with a fabric spray (the type used for tents, shoes or furniture). Most likely you will see a difference right away. During rainy weather the sails will shed water better, keeping them lighter and better shaped, and that can really make a difference in your boat’s heavy-weather performance. The fabric treatment also makes the sailcloth more pliable and easier to flake and fold. It’s a cost-effective performance enhancement to lavish on your boat.


Has anyone tried this?  What sort of fabric spray? Waterproofer?