Baltic birch is prone to "raised grain". Thoroughly wet the surface, dry, sand lightly with 180 / 220 grit, sometimes I do it again. A light coat of shellac / sealcoat, then 400 grit sanding leaves a smooth surface. It is a pain, but when building drawers, I can finish an entire plywood sheet and not finish the drawer at all. I have also used lacquer instead of shellac with equally good results.