Contrary to the near grains of walnut and birch, the wide-open grain of red oak is difficult to fill with complete alone. If you don’t fill it with paste filler, the shapes of the grain will be visible, even if you employ several end coats, and the surface won’t ever be perfectly horizontal. Paste filler is not a kind of putty. It is a pourable material made from varnish and fine aggregate. It is inclined to change the color of the wood, so if you would like to preserve the natural pine color, it’s best to seal the wood prior to applying it.
Pick a paste filler which matches red oak if you don’t plan to stain the timber. If you are going to use a blot, pick a filler which matches the stain color.
Seal the wood by massaging in a very dilute solution of varnish and mineral spirits using a rag or brushing on a thin coat of shellac. Let the sealer dry before proceeding. Omit this step if you plan to stain the timber. If you are staining, apply the stain after you have sanded down the paste filler.
Pour a small amount of paste filler on the surface and spread it with an old paintbrush. Work it in the grain using a charge card or even a plastic paint scraper.
Wait for the filler to turn dull, which means it’s partially hardened, then rub across the grain with a rough cloth. When you have removed most of the excess filler, wipe the surface off with cheesecloth.
Let the filler dry immediately, then shine a work light onto the surface and look for open pores or areas which are not filled. Apply another coat of filler if needed.
Sand the surface with 220-grit sandpaper, moving the paper along the grain of the timber, before you blot and finish the wood.