I used a painters tarp for my first fly. It leaked a little until it got wet and then sealed itself. At one time I tried to waterproof it using a 3M spray can product. It was expensive and not very effective. Later I used a product called "canvac" to waterproof my tent when it started leaking. I got it at "The Log cabin Shop". It is a bit pricey, but effective. Other People have used Thompsons on their lodges, but it tends to get stiff if laid on heavy. I would never spray waterproofing. It doesn't get the penetration or coverage you need. My friend sprayed canvac and it was running off his tent and didn't work. I rolled a 10x15 wall tent with a disposable roller used less than half a gallon and never had a problem.