I just used a product called Rustoleum NeverWet. It’s a two step process. I used it on a Schaffer’s nylon slicker I’ve had since forever. It seems to work pretty well. It’s advertised to work on suede, leather, cloth, etc. I can’t comment on other cloth uses.
It does leave a slight “residue” in that it gives the item a sort of very pale whitish/grayish tinge. It certainly does not change the color and unless you’re looking for it you likely won’t even see it. But it is there. For a rain sheet, slicker, work boots, etc. I’d not think that was a big issue. I’d not use it on nicer clothes without more experimentation.
I’ve had poor luck with Scotchguard.
In reality, if you want “waterproof” you’ll have to have something that won’t breath real well or drop some really big bucks on Goretex or the like. For most uses the best you’ll get is “water resistant.”
G.