So I decided I would try this out on my used-to-be waterproof tailbag.
It DEFINITELY works, though it looks and feels kind of odd afterwords. (like wax)
I think I may have used too much, or didn't dry it out enough, but it's waterproof.
I had a little candle made of beeswax, so I pulled out the wick and melted it in the microwave in a ceramic bowl. It took like five minutes or so.
I painted it onto the tailbag using an old paintbrush, it sucked it up, but it dried out into a layer of wax. Then I took the hairdryer to it, and melted it all down again, it started to go away. When I was finished, I went to open the bag and it was stuck together -__-
Stupid me, didn't put news paper INSIDE the bag! So I stuffed it up and then went over it again with the blow dryer. Now it looks a lot better.
If you ever want to do this, just do the bare minimum first. It took my almost 45 minutes to get it to look not like a coating of wax xD
(if it's a velcro bag, don't get the wax in the velcro. I did, and it's still not all out!)