Outback Trading Co. manufactures a large variety of oilskin coats that are designed to repel water instead of absorbing it. However, if the coat is put away while wet or is subjected to moisture over an extended period of time, mould will start growing on the coat's material and will likely cause an unpleasant and musty smell. Remove mildew from Outback coats as soon as you discover it to prevent odour.
Hose down the jacket with regular water and then scrub at the mildew with a clean, damp sponge. Avoid using detergents since they could break down the oilskin's treatments. The water pressure of the hose combined with the sponge should remove most mildew from the coat.
Mix 3 parts warm water with 1 part white vinegar in a spray bottle. White vinegar is an effective cleaner and mould remover. The water dilutes it so it won't damage the oilskin.
Spray the mildew stains and allow the solution to sit for a minute. Scrub the mildew with the scrubbing sponge. Repeat until all the stains are gone.
Hang up the coat to allow it to dry completely before putting it away.
There are oilskin-specific cleaners on the market that remove mildew if you don't wish to use the vinegar solution.
Test the vinegar on a small area of the jacket to verify it won't harm the surface. Add more water to it if it appears to affect the material.