Absolutely! Unless you're using a paint and primer all-in-one, a primer is absolutely essential to give you the results you're looking for on your deck. Primer helps give you an even and smooth coat, but that's not even the most notable perk. Primer helps the paint adhere to the surface you're painting, so if you skip the primer, your paint likely won't last nearly as long, and it'll start chipping sooner rather than later. This is especially true in humid environments, but it can happen anywhere. So, whatever you do, don't skip the primer!