To get a smoother finish, density is important. If the poly is too thick, you will find it difficult to get maximum coverage. That’s why you have to thin your poly to an extent where the flow of application gets smoother. In terms of water-based poly, do it with caution. Because water-based poly comes thinner most of the time. Another important thing to get a smooth finish is to prepare the surface. If the surface is not well suited or uneven or has any remaining of previous finishing elements, then getting a smoother finish will be tough. So make sure that you prepare the surface by sanding it properly.