I often receive e-mails with the subject title "In search of a glass smooth finish" or something like "Can't get level finish on oak". The problem lies in the type of wood you are working with. It's much easier to obtain a glass smooth surface on some woods like maple, cherry and birch, because the pores in these woods are relatively small and uniform. Therefore, when you apply a topcoat finish such as lacquer, varnish or poly, the first one or two coats will usually be enough to fill the pores and level the surface. After that, it's just a matter of applying a few more coats to build the finish up enough to obtain enough protection and depth.

Other woods like oak, ash, mahogany and walnut have pores that are larger and not as uniform, therefore even if you apply several coats of finish, it will probably not be enough to fill the pores to the point where they are level to the surface of the wood. However, some finishers do use the technique of applying one coat of finish, letting it dry, then sanding it back down, applying another, sanding back etc.... until the pores have been completely filled by the finish. This technique is labor intensive and time consuming, but I do use it in certain situations for example: when surface I am finishing has contrasting colors, whether it has been stained with different color stains or has inlay work or just different types of wood with contrasting colors.

Filling The Pores With The Finish

Many finishers use a commercial sanding sealer to fill the pores. I don't recommend this. Sanding sealer should be used to make the first coat sand easier. The sealer will stiffen wood fibers so that they will stand up for sanding. Most commercial sanding sealers contain zinc serrate (a mineral soap). This makes the finish easier to sand. The serrate makes the finish easier to sand by softening it and here lies the problem. If you use multiple coats of sanding sealer to fill the pores you will weaken the overall finish.

I have found that the best thing is to use the finish itself to fill the pores. Take some of the finish you are going to use and reduce it about 25 percent with the proper solvent. If you are using oil based varnish. or oil based poly, use mineral spirits. Next, apply a number of coats, letting each coat dry and then sanding back with 220 or 320 grit paper wrapped around a flat cork or wood block. If you have stained the wood, be careful not to sand too much or you will cut into the stain. Here it is best to build up a number of coats first before you start to sand. When the finish in the pores builds up to the same level as the surface, the pores have been completely filled. To make sure the pores are filled, shine a light down so it reflects off the surface at about a 30 degree angle to the surface. Look for any small pits. If you see any pitting, the pores have not yet been completely filled, go back and apply more coats. Once no pitting can be seen the pores will have been completely filled. You now can apply your finish.

Filling The Pores With Paste Filler

Paste filler is a much quicker way to fill pores than using a finish. Do not get this product confused with wood fillers or wood putties, it is a completely different type of filler. Unlike wood putties, paste filler should not be used to fill nail holes or cracks in wood. Also called grain filler, paste fillers contain some type of binder like varnish and finely ground quartz like silica to add bulk (so it will fill quickly) with talc sometimes added and a pigment to add color. You can purchase paste filler in several colors, natural (which is similar to a light tan like maple), walnut, red mahogany, brown mahogany, dark brown oak, white, black and more. You can also tint the filler by using japan colors (if you use an oil based filler) or water colors (if you use a water based filler). Paste filler is not clear so if you are filling a dark wood don't use the natural and expect it to blend in, it will not work. It is best to use a filler as close to, or tint the filler to the desired color. If you want to accent the grain a little, make sure the filler is slightly darker than the color of the wood. This will make the grain stand out more.

At What Point Should You Fill

It really depends on two factors. Is the wood going to be stained? If so, what type of stain, pigmented or dye stain?

If you are not going to stain, first, apply a wash coat of whatever finish you are planning to use. Take the finish and reduce it 50 percent with it's suggested solvent and apply one coat. Let dry, but do not sand. This will seal the wood and prevent it from changing color too much when you apply the filler, especially if you are using a filler that is darker than the wood. The pores once filled, will be the color of the filler but the wood should only be slightly darker. You can also use the filler as a stain by not applying a sealer coat. The filler will not only fill the pores but also color the wood slightly.

If you are planning to use a pigmented stain, it is best to fill first, then stain. If you stain first, then fill, when you rub off the excess filler you will also be removing much of the pigment.

I believe it's best to use a dye if you are going to both stain and fill. There is less chance of the dye and filler bleeding together and the dye will not obscure the grain as a pigment will. First, stain the wood using an aniline dye or NGR stain. Next, apply a wash coat (same as described above). Let the wash coat dry, but do not sand, then apply the filler. This method will enhance the contrast between the pores and surrounding surface.

Applying The Filler

One of the keys to the application is to make sure the filler has the proper consistency before you apply it. Most fillers come pre-thinned and ready to use. However, others still need thinning. The filler should be thinned to the consistency of heavy cream. If necessary, thin slowly, adding thinner a little at a time and stirring well. The filler should be just thin enough to flow off the stick. You can thin oil based filler with either mineral spirits or naphtha, water based filler should be ready to use, but if you need to thin use water only. If you have never worked with paste filler before, I suggest you start with the oil based type and thin it (if needed) with mineral spirits. The oil based filler thinned with mineral spirits will set up slower and give you more time to work. If the filler sets up too quickly and sits on the surface too long, the excess will be very hard to remove and you will have to dampen a rag with solvent to remove it. This often pulls the filler out of the pores and you have to start all over again. Thinning with naphtha makes the filler set up quicker, only use this solvent once you get used to working with filler, it will speed up application time. Water based filler sets up very quickly.
 

Coarse cloth like a piece of burlap is perfect for removing excess paste filler from the surface once the filler has set up.  You can purchase burlap through several mail order supply stores or cloth suppliers online.

Apply the filler with either a old stiff brush or pour it on and work it down into the pores with an old credit card or piece of plastic laminate. Work on small sections about 2 ft. square. Do not try to apply filler to the whole piece. By the time you finish, the filler would have dried so hard, that it will be very hard to remove. Let the filler sit until it hazes over (turns dull). Then, using a coarse cloth like a piece of burlap, wipe off the excess working against the grain, trying to cut the filler off at the surface. Once most of the excess is off, take one last pass with a piece of lint free cheesecloth lightly wiping with the grain. Then let the filler dry overnight. The next day, inspect the surface by shining a drop light down onto it. You should be able to detect any filler that has remained on the surface. Using 320 grit paper, lightly sand with the grain to remove any of the remaining filler that is on the surface. The light should also detect any pitting. If you see any pits, this means the pores are not completely filled. You will have to apply the filler a second time in order to completely fill the pores. This is rare and only needed when the pores are exceptionally large.

After the filler has been applied, you may then proceed with the application of your finish.

IMPORTANT NOTE! PASTE FILLER SHOULD ONLY BE USED UNDER TOP COAT FINISHES SUCH AS LACQUER, POLYURETHANE, VARNISH, WATER BASED FINISHES ETC.  DO NOT USE UNDER PENETRATING FINISHES SUCH AS OIL VARNISH BLENDS LIKE WATCO DANISH OIL, TUNG OIL, OTHER PURE OILS OR THIN OIL WAX FINISHES.

PLEASE TEST ALL THE ABOVE PROCEDURES ON SCRAP BEFORE YOU APPLY THEM TO A GOOD TO A PROJECT.