I am building a computer desk and need to know if I should fill the grain on the top. It will be made with oak faced plywood that is edge trimmed with solid oak. I am confused about how and when you should fill the grain. One book shows the author using Danish oil and sandpaper and sanding the top before the finish dries. Will this affect the water-based stain that I am using? When should I fill the grain and how should I do it? Does it make a difference if the finish is water-based or oil-based? What about just building up the finish with polyurethane?
Michael Dresdner: First things first. No, you do not need to fill the grain on your oak computer desk, and unless you have a compelling reason to do so, I would not advise it.
Pore filler allows you to create a perfectly flat (and rather thick) finish on porous wood. However, our current style sensibilities encourage us to let wood, even under a finish, look like wood, and open pore finishes are far more popular than filled pore finishes. This is especially true of woods like oak, which are very difficult indeed to fill. Still, it is entirely a matter of taste, and therefore is up to you.
As for the rest of your questions, (all of which are predicated on the assumption that the answer to the first question would be “yes”).
If you choose to create a filled pore finish, you should fill the grain after the first coat of sealer. That means dye stains must go on the wood before the sealer and pore filler, and pigment stains may go on after the filler has been sanded. And yes, you may have compatibility and adhesion problems if you put water-based coatings over oil based pore filler, though not the other way around. Finally, yes, you can fill pores by sanding down successive layers of polyurethane, or by sanding coats of Danish oil, but that is certainly the slow and tedious way to achieve something that is really not that difficult when done right.