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).
Some waterproofing sprays should be applied while the raincoat is wet and others should be applied when it’s dry. Refer to directions on the product you are using for specific instructions (See Reference 1).
If your raincoat has a DWR finish, you can often restore this just by washing and drying the raincoat according to the care instructions. The heat from washing and drying the coat reactivates the DWR molecules. Avoid using liquid soaps, fabric softeners or dryer sheets when washing raincoats with a DWR finish because this can interfere with the water-repellent qualities (See References 2 and 3).
You can also run a warm iron over the coat to reactivate the DWR molecules.
Plastic raincoats do not need waterproofing (See Reference 3).
Make sure you are in a well ventilated area when spraying the garment, and be careful not to get the spray in your eyes.