So, you've just finished that stellar table top and you'd like to give it a 'smooth as glass' finish. What's a woodworker to do? Why, 'Fill and Finish' of course.
There are two kinds of 'fillers' - putty type fillers used to fill scratches, dents, and holes in wood, and grain (aka pore) fillers that serve to level out the surface of open grained woods. It's the latter filler that concerns us here.
Woods such as oak, ash, elm, mahogany, chestnut, walnut, wenge, and teak are characterized as having 'open grain' because the wood pores are large. In contrast, 'closed grain' woods like hard maple, cherry, poplar, beech, and bubinga, have much smaller pores.
You don't have to fill the pores of any wood before applying a finish - and a lot of woodworkers don't. It's a matter of what you want the final surface to look (and feel) like. If you want to achieve a super smooth finish on open grained woods fairly quickly, then your best bet is to fill the pores before you apply your chosen finish. Good candidates for pore filling are large horizontal surfaces - the tops of tables, desks, sideboards, dressers, fancy boxes, and the like.
Sealing is Not Pore Filling
A sealer coat, washcoat, or a sanding sealer won't necessarily fill wood pores.
The first coat of finish you apply to a wood surface effectively seals the wood, which is why it's called a 'sealer coat'. However this coat doesn't necessarily fill up all the pores - especially on those open-grained woods I mentioned above.
When you apply that first coat of finish (i.e. the sealer coat), the wood fibres will likely swell, giving the surface a fuzzy texture (known as 'raised grain'). This is especially apparent with water-based finishes. Before you apply subsequent coats of finish you'll want to sand off these fibres. Once you've sanded back the raised grain, it won’t occur again.
To make it easier to sand back this first coat of finish, many woodworkers will thin the finish with it's appropriate solvent, or use a 1 pound cut of shellac. This thinned version of the finish is called a 'washcoat'. In production shops, where they often spray on lacquer or varnish, they'll typically use a commercial product called a 'sanding sealer' rather than a washcoat.
Oil or Water
There are two basic types of grain fillers - solvent (or oil) based, and water-based. You'll want to use an oil-based filler only with an oil-based finish. However, you can use a water-based grain filler under either an oil-base or water-based finish. I use water-based fillers all the time now. They don't emit toxic fumes, making them safer to use, better for my health, and better for the environment, and they're easy to apply and compatible with any finish. The only issue is that, as with all water-based products, they dry fairly quickly. The water-based filler I've recently been using is Aqua Coat's Clear Grain Filler.
If you'll be finishing with a penetrating finish (tung, linseed, wiping varnish, or oil/varnish blends such as Danish oil), it's best not to use a pore filler, as these as they don’t cure hard enough.
Colour or Not
If you plan to stain the wood surface, you can do so before or after applying the filler. The fillers themselves come in clear, natural, and some colours. You can add a colorant to the clear or natural fillers. Regardless, the filler will likely take the stain differently from the surrounding wood, and sanding after you stain might result in some of the stain being removed. To avoid disappointment, it's best to do some test staining on scrap wood cut-offs before you commit to your project.
Easy to Apply
The water-based filler that I currently use, Aqua Coat Clear Filler, is very easy to apply. The only caveat is that you have to work in small sections at a time, and reasonably fast. Though the process is quite simple, I'd suggest you begin by sealing and filling a test piece or two - particularly if you've never done this before.
I almost always sand with a random orbital sander, up to 180-grit, followed by hand sanding with 220 grit, in the direction of the grain. After wiping the surface clean with a tack cloth I apply a seal coat. Because I use a fair amount of shellac, and usually have a batch laying about, I seal the surface with a 1 pound cut of blond shellac. I let the shellac dry for a couple of hours and then lightly sand the surface with a piece of used 320- or 400-grit sandpaper. Just kiss the surface to remove any bits of raised grain and dust nubs.
You don't need to mix Aqua Coat Clear Filler before using it. Just scoop a tablespoon or so and drop it on the work surface. The objective is to pack the filler into the pores of the wood. The easiest way I've found to do this is with a rubber squeegee. I suppose you could also use an old credit card. Work the filler diagonally to the grain and don’t forget to do the end grain. Aqua Coat spreads easily, and I find that I can do about one square foot of surface in under a minute. I've never felt the need to thin the filler with water.
Using a squeegee makes it easier to control the filler, and usually I end up with a very thin, even coat spread over the work area. You can easily see if all the pores are filled by looking at the surface at an angle, against a bright light beamed obliquely to the surface. Once the surface begins to dull I wipe it down with a cloth, across the grain, just enough to remove any bits of paste that haven't gone into the pores. I'm not overly concerned about getting every single micrometer of space filled, as I always lay on a second coat of filler after the first coat has dried for at least an hour. Before repeating the process I'll lightly sand the surface with 320- or 400-grit paper. While I like to let the surface cure for a few days before applying the finish, I don't think it's absolutely necessary.