The deep grain lines in woods like oak or walnut will telegraph through the clear finish, no matter how many coats you apply. And that’s fine; it’s part of the character of coarse-grain woods. But if a perfectly smooth surface is the look you want, use a grain filler. You’ll find several products online or at woodworking stores. With most, you wipe on the filler, squeegee off the excess with a plastic putty knife and then sand after it’s dry for a smooth-as-glass surface.