Re: how to waterproof boat cover
I think it would take major effort to truly waterproof an old cover; better to replace it or accept it. However, you might drop by an awning/tent maker and see what they say. I learned to waterproof duck canvas for a kayak with house paint from an awning guy--who had built lots of kayaks himself.
I have fiberglass boats, so as my covers got leaky, I treat them as dirt, sun and leaves covers. They shed major water. but the water that does seep in, nice and filtered, runs out the open drain hole, no harm done.