Using a high-quality beard balm is a great way to get rid of patchy beard. A beard balm basically does two things: it makes your beard easier to manage and it provides important nutrients for your beard.
With the help of beard balm, you can actually move longer hair over patchy areas. Sort of like combover, but for beards. Also, if you chose a quality beard balm, like The Cruise Control from CPH Grooming, it will contain nourishing ingredients to help your beard thrive, such as essential oils, shea butter and other critical ingredients.
In short, fill in patchy beard with a beard balm by covering patchy areas, and provide your beard with critical ingredients that it needs to grow optimally.