MDF is waterproof and can be used in places with a lot of moisture, but you shouldn't use it outside. If you want to use it outside, you should make it more waterproof first. No matter what method you use, you should know that MDF is resistant to moisture but not to water or to being waterproof. It can still swell and warp if it directly interacts with the elements. Painting the MDF board is the only way to make it waterproof.