I burnish all my bows, back and belly, for three reasons really. First, it creates an exceptionally smooth finish that can't be matched by sanding alone. Next, I feel that it provides some protection against splinters raising. And finally, the compressed cells form a natural moisture barrier that serves as a second level of defense beneath whatever coating you wipe or spray on top. If you place a drop of water on the bare wood of an unburnished bow, you'll see that it soaks in on contact. Do the same thing on a burnished bow and the water will bead. Similarly, if you dye a burnished bow, the grain won't raise.
