If you want the natural look of the pores in oak and a smooth none plastic finish without using a wood filler, this is what I do:
1. Sand to 220 or 320. Typically 80, 100, 150, 220, 320 grit with a random orbital sander (ROS). I use my sanding box. The ROS makes for Quick sanding and a beautifully clear, smooth surface!
2. Formby's Tung Oil Varnish finish or 50/50 Spar Varnish/Naptha (or Mineral spirits) mixture.
3. Vaccuum and tack rag with a mineral spirits soaked rag. Mineral spirits will give a preview and show problems like glue. This will give you a chance to correct before doing the finish.
4. Stain if desired.
5. Wipe on finish with short hair pad. My theory is the short hairs are pushing the finish into the pores and thouroughly wiping the walls of the pore, resulting in more finish filling up the pore, more so than by any other applicator. It won't be full, but fuller. Subsequent coats and other technique described below will fill it up.
6. 1/2 to 1 hour minimum to next day - lightly sand with 3M finish pad. For the Formby's varnish, this step is to clean off dust or irregular buildup. In later coats you may wish to skip the 3M sanding. But for the Spar or other Polyurethan Varnishs always 3M sand between coats. It serves the additional purpose of scratching the surface for the poly layers to bond to each other.
7. Wipe with mineral spirits rag. If the pores are real deep then sand with 320, vaccuum and wipe again. Otherwise go to the next step.
You may choose to always do this step after the 1st coat rather than step 6. What this step does is lower the build up that is on the surface without lowering the build-up in the pores. Sand until all the sheen is gone. You are at the top of the pores here, don't sand any deeper. Repeat this sanding operation at anytime the pores seem to be deep. Most likely this is the only time necessary.
8. Apply 2nd coat of finish, 1/2 to 1 hour minimum to next day cure time, then 3M pad sand, mineral spirit wipe.
Be sure to use the 3M type of finishing pad or hand sand 320 grit sandpaper. Do not use the power sander in this operation. It will result in swirls or scratches.
9. Wipe with zylene using a rag. The threads of the rag will help smooth and capture dust.
You should be seeing a smooth surface forming. The zylene causes the surface of the varnish to reflow. I believe it is wiping varnish from the surface and filling the pores some more. You will immediately see the difference.
10. Repeat coat, 3M, mineral spirit wipe, xzylene wipe as desired.
If at any point it starts to look like plastic go back to 320 grit ROS or hand sand to clear it up. If you use the ROS on the 2nd or later coats, reduce the speed. You don't want to cut too deep.
Always use a respirator and ventilation while applying the finish, mineral spirits, and especially the xzylene. Use the respirator or a dust mask while sanding. Protect yourself and your family!
After doing the 3M pad sanding, vaccuum then rub your hand over the surface. If you feel dust specs, can't see them but can feel them, then lightly go over it with a paint type of scraper. Don't use a shaving scraper or plane. You just want to knock off the fine specs. Scrape lightly without scratching the surface.