Waxed cotton field garments come into their own in places like grouse coverts and wetlands, protecting outdoorsmen and outdoorswomen from brush and precipitation. Naturally, they get quite dirty in the process. Clean yours first to avoid spending more than what you have to when you send it to Wildfell for reproofing.
Before going further, please be aware: Never dry-clean or machine-wash a waxed cotton garment. These methods will strip it of all residual wax and likely cause it to shrink and/or warp. Now, what about the right way to get the dirt off?
To clean a waxed cotton garment, reiteratively scrub it with a nylon bristle brush and hose it with room temperature, or cooler, water. Wipe down its shell with an absorbent towel afterwards. Then, air-dry it in a well ventilated space.
The garment must be fully dry before it is packaged and shipped. Carefully inspect pocket interiors and other tight and unexposed parts to verify that they are indeed moisture-free. Otherwise, mildew will develop. It’s a bit like cleaning that 12-bore in that nothing can be overlooked, not even the choke tubes.