Raincoats are designed to keep you dry, and most are treated with a waterproofing agent to make that happen. Over time, the waterproofing protectant may wear off. Plastic raincoats will naturally keep water out because of the material, but other raincoats such as those made of nylon will need to be waterproofed. If you buy a waterproof coat or jacket, the protectant on it is good until you notice the water is no longer beading up on the fabric and instead is soaking through to the inside (See Reference 2).
Purchase a can of waterproofing spray made for outdoor gear. If possible, choose a product with durable water repellent (DWR) qualities, as this gives the best protection against water. A product that comes in a spray can is easiest to apply to a raincoat (See Reference 1).
Wash and dry the coat according to the care instructions. This will vary greatly among manufacturers. If your raincoat is plastic, brushing off loose dirt with a rag, then wiping it down with a clean rag and warm water will suffice. Hang the raincoat on a clothes hanger once it is clean (See Reference 2).
Shake the can of waterproofing spray well. Hold the nozzle about five to eight inches from the coat (See Reference 1).
Spray the protectant onto the coat using long, even strokes. Once the entire coat has been sprayed, put it in the dryer on medium heat for one hour to dry and to lock in the protectant (See Reference 1).