While it’s true that trapped moisture can promote the growth of mold and make your grill rust, usually it is low-end covers or user mistakes that are the bigger problem. The best grill covers will not only keep moisture from getting in, but they will allow it to get out via a feature such as a vent, which allows air to better circulate. To prevent rust, it’s extremely important that the grill and the grill cover are both completely dry before covering and your grill is not stored in a shaded area.