Oilskin / Waxed cotton is exactly that! Cotton fabric made waterproof using a mixture of oils and wax - called Ducksback.
Oilskin is what the cattlemen use time and time again because they know.... it lasts the test of time.
Fair Dinkum Dogs has been making Oilskin dog coats for 27yrs and we have many reports of our coats still in use 15yrs later.
This is because with a reproofing using Ducksback they can come up like new.
Ducksback when cold looks like wax. We have found the easiest way to apply is to pop the item you are reproofing and the Ducksback tin in front of a heater to warm up.
Go make yourself a cuppa.
Once the Ducksback is melted use a cloth or a paintbrush to apply to the garment. Don't overdo it - less is best, applying a thin layer over the entire surface.
Once applied leave your garment in a warm (not hot) area so the fabric can soak up the applied Ducksback.