Waxed cotton occasionally needs a refresh. That’s a good thing! When your waterproof breathable whatever needs a refresh, you have to dump DWR (durable water repellent) in your washing machine, which is not great for the ole environment. In the case of waxed cotton, you simply have to add some more food safe paraffin, which not only is non-toxic, but it’s also a waste product that’s being repurposed. To use duck wax, simply heat the tin in a double boiler of water, then apply with a $1 bristle brush. Rub it in with an old t-shirt. Use a hair dryer to get it thin and smooth. Sounds like a lot of work, but you could rewax a bag in 30 minutes or less. You need less wax than you’d think, and probably not every bit of the bag needs new wax.
If you want to wax a bag from scratch, you’ll need a few tins of wax. I’d get 3, and have the 3rd just in case. Unwaxed cotton soaks up a ton of wax. In general, we don’t recommend waxing stuff from scratch. It works, but it takes a few months to sort of bed in and not be all oily.