Some fire pit covers are already constructed to be waterproof, but there are steps you can take to waterproof one yourself. One option is to buy a spray treatment like NeverWet. This product (by the creators of Rust-Oleum) includes a base coat and a topcoat. This type of multipurpose spray helps metal, including fire pits, repel water, mud, ice, and other moisture. It helps reduce corrosion and increases the longevity of your fire pit. This is not the same as entirely waterproof, however, and it also needs to be applied more than once.