I once walked with a fellow who had been a fabric finisher in an earlier life. This was someone who determined what chemicals were used to give the raw woven fabric the right characteristics.
His advice was to get something made specially for the fabric you wanted to proof. So if it is cotton, get something made for cotton. The reason is that the proofing has to bond to the fibres of the fabric, and something that bonds well to nylon may not bond to cotton.
He was also quite skeptical of Scotchgard, which is just a version of Teflon. Teflon's main claim to fame is that things don't stick to it. By the same token, it does not stick well to things it is applied to. In my experience, Scotchgard was good for a few wettings, but got less effective each time.
Roger Caffin has some good information on waterfproofing and fabrics on the NSW Confederation site:
http://bushwalking.org.auLook for the FAQ section.
This post is copyright by davidmorr. Permission to reproduce elsewhere may be granted on application. Please PM me for details.