About 2 years ago I shot a quick video on silicone spray application.
For the video I used the McNett version (possibly the Atsko re-packaged...)
Note that you have less than 2 oz of silicone in the can, the rest is the propellant.
https://www.youtube.com/watch?v=GBnkGIz9cco&list=UU0PuLUKvG7Fxxex5BMVK4vw&index=42
Why no detergent ?
From Hilleberg :
If you tent is dirty try and clean it with a wet cloth or sponge in luke-warm water or rinse it in a bath tub. Do not use any detergents and do not wash your tent in a washing machine!

(Hilleberg use silnylon for their tent fabric)
From McNett:
Dirt, grime and household detergents leave residues that damage water repellent fabrics, leading to absorbing rather than shedding water.
From Tarptent:

Tarptents can be hand washed in mild soap and water. We use and recommend WooliteŽ Fabric Wash.