Can't imagine this working. Windbreaker clothing already have DWR coating, which Scotchgard basically is, and even if they cohabited the fabric harmoniously you wouldn't receive double-durable water repellancy. Light moisture beads up and runs off; heavy moisture/windblown rain wets through.

Waterproof means either impermeable fabric or a bonded waterproof layer. Cheap, impermeable raingear is easy to find and doubles as a great portable sauna. Waterproof-breathable fabrics are trickier and come in many types, the most famous being Goretex. Those can get expensive, but there's always sales and second-hand. Somewhere off to the side are ponchos, which can protect you and your pack and generally are pretty cheap. Make a decent tarp in camp, too

When I was in college it was the rage to empty a can of Scotchgard into a pair of Levis and go skiing. What can I say?

Cheers,