Because we were putting our new table cover over our existing table, we needed a rim that would hold it in place that was made of 1x2's. The 1x2's are an inch and half wide so adding two of those adds three inches overall to the length and width. I also added an extra 1/8" overall to both the length and the width so that there was a gap underneath so that the table could easily come on and off.
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.