If you want to finish these woods to a super smooth surface, then you’re going to have to fill the pores so they are level with the surrounding surface of the wood.
There are two ways to fill the pores. On close grained wood you can use the finish itself. Essentially you apply as many layers (or “coats”) of finish as needed to fill the pores, either sanding between each layer, or sanding after applying the last layer of finish.
On open pored wood your best bet is to use a paste wood filler. There are two kinds of fillers, one for film finishes (shellac, lacquer, varnish, polyurethane) and one for water based finishes. Keep in mind that you shouldn’t use a filler under a penetrating finish (tung, linseed, wiping varnish, or oil/varnish blends such as Danish oil), as they don’t cure hard enough.
For a detailed discussion of pore filling you might want to read the ultimate reference book: Understanding Wood Finishing by Bob Flexner (ISBN 0-7621- 0191-1).
Filling Is Not Sealing
You may come across a product called a sanding sealer. This isn’t the same thing as a pore filler. When you apply your first coat of finish to a wood surface, the wood fibres will swell, giving the surface a fuzzy texture (referred to as raised grain). Before you apply subsequent coats of finish you need to sand off these fibres. Once you have sanded back the raised grain, it won’t occur again.
Because finishes such as varnish, lacquer, and water based finishes are difficult to sand, manufacturers have created “sanding sealers” which are applied before the finish. They raise the grain, but are very easy to sand, which makes them a popular item in production shops. Sealers don’t fill the pores then, they raise the grain. If you plan to use a sealer, do so after you have filled the pores.
Colour Your Wood
If you plan to stain the surface, you can do so before or after filling. Fillers come in clear and tinted colours (or you can add a stain to a clear filler). 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. Don’t forget that over time most woods will darken somewhat.
There are a lot of pore fillers on the market. I use a clear, acrylic filler (Fuhr Paste Wood Filler, product code 175). After sanding the wood surface to 180 grit with a random orbital sander, I hand sand with 220 grit, then wipe the surface clean. I then assemble my filling kit.
Laying It On
Begin by mixing the filler thoroughly. You can apply it by brush, roller, or rag. The objective is to pack the filler into the pores of the wood. The best way to rub it in is with a rubber squeegee. For a small surface, a paintbrush works well. Work the filler diagonally to the grain and don’t forget to do the end grain. Work it in just until it gets somewhat pasty. It shouldn’t take more than three or four minutes to work the filler into a one square foot of surface.
Wiping It Off
The filler has a glossy sheen that begins to dull after a few minutes, then rapidly begins to dry. I find that the Fuhr product dries very quickly, so you should remove the excess paste before the filler becomes too hard as it’s very difficult to remove then. For the first few times you try pore filling, be on the safe side and wipe it off sooner than later.
I begin by scraping off any excess with a laminate chip or an old credit card. I tend to wipe in the direction of the grain. Then I take a piece of burlap or coarse cloth (which ever is handy) and rub the surface thoroughly.
On a molded or turned surface you can use an old toothbrush or sharpened wood sticks to remove excess filler from details.
Once you’ve got the excess filler removed take a break and do something else. You’ll want to give the surface several hours to dry.
Sanding it Back
Once the filler is completely dry it needs to be sanded. The idea here is to remove all the filler from the wood surface except from the pores.
After it dries, check to see if the pores are filled. Hold the surface up to a raking light (i.e. hold the board at a shallow angle to a light source so you can better see the surface of the board). If the filler has shrunk in various places, you can put another coat of filler over the piece. Otherwise, you’re now ready to put on a finish.
There are two keys to success in pore filling: practice, and practice. Don’t worry about rubbing the filler off too soon; you can always put another coat on. Do give the filled surface ample time to dry before you do the final sanding and, once filled to your satisfaction, let the surface cure for a few days before applying your final finish.
Next issue Carl will cover oil finishes.