I’ve had a couple people ask me my technique for filling the open grain of wood with a contrasting color, so here it is, with a few pictures.
First, grain filling is usually only necessary when working with a wood with very open grain. Ash and oak are good examples. But sometimes you will want to fill the grain on a wood like mahogany or khaya. It’s the same technique in both cases.
First is to get the wood sanded smooth, and then to thoroughly wipe off the sanding dust. If you’re filling the grain of the wood with a contrasting color, having that grain partially filled with wood dust from the same wood will lessen the effect. That doesn’t mean that’s not an appropriate technique in some cases, it’s just not what I’m describing here.
If you see that some of the pores still have dust in them, or perhaps even if you don’t, brushing (with the grain) can remove even more of the dust from the pores, leading to a better fill. Both a nylon brush and a wire brush will work, though the effect is different. Thanks to @firstname.lastname@example.org for suggesting using a brush, which I forgot to mention here on the first edit.
Next is to mix up the grain filler. I use either black or white Wunderfil by Rockler, and then add dyes to the white if I’m after a colored fill. I dilute the Wunderfil with water 50-50 so it’s fairly thin and will flow into the pores of the wood. Some wood fillers need to be diluted with oil, which isn’t as easy to use. Today I used blue milk paint mixed with white Wunderfil. I mixed up the milk paint normally, then mixed that with white Wunderfil with about equal parts of each. Then smear the grain-filler on the wood. I generally put on blue gloves and use my hand, but if you’re working on a flat surface, you can use a squeegee. The important thing is to go across the grain so the fill is pushed into the holes and not wiped back out.
In many cases, after you give the fill a few minutes to dry, you’ll notice that the grain has opened back up, and can take more fill. If that’s the case, apply a second coat over the first. I almost always need a second coat, but it really depends on the wood and what kind of fill you’re using. Safer to apply a coat you don’t need than to leave some pores unfilled, though.
With the second coat on, you want to wait for it to dry. A half hour is generally enough with Wunderfil, but more time won’t hurt. Once it’s dry, you sand back to bare wood, but be careful not to overdo it and sand away all your fill, too. I generally will sand with 400 grit sandpaper, but finer will work too.
At this point, you can begin finishing. I’m going to use a homemade friction-finish made from shellac and tung oil on this pen, so I’ll apply my first coat of tung oil and then wait overnight before moving on to the friction finish. This will give the fill and oil a chance to cure a bit, and makes it less likely the finishing process will pull the fill out of the pores.
You can see how the oil brightened up the colors. If there are spots that aren’t filled completely, you might need to apply more fill at this point (and then sand it back again later), but I generally try to do any more filling before putting on the oil. Sanding back will be messier and the fill may not adhere very well to oiled wood.
Once the blank has had time to cure (at least overnight with tung oil), continue with your finishing regimen. For me, that’ll be two or three coats of an oil and shellac mix, applied at the lathe, and buffed with a cloth so the heat speeds the curing of the oil. That finish will get another day or so to cure before I handle the piece too much.