When you apply a clear finish, your brush makes a difference. A cheap, throwaway model can cause problems, while a quality brush that suits your particular finish will help you achieve first-class results. You'll spend more upfront, but that's not a decisive factor; take care of it properly, and a good brush will last much longer than a cheap one.
The photo above provides a cutaway view of a finishing brush made by the Elder & Jenks company, brushmakers since 1793. The bristles are set into a slightly convex base of epoxy and then trimmed to complete the chisel shape. Double dividers give the bristles extra support. They also create reservoirs that hold a small amount of finish, feeding the brush as you use it.