3 Easy Steps to Grain Filling
There are both oil-based fillers and waterbased fillers – you can use the former only under oil-based finishes, while you can use water-based fillers under any kind of finish (with the exception of oils or oil blends, like tung oil, boiled linseed oil, or Danish oil). I’ve used two brands of grain filler with good success: Bartley fillers and Mohawk fillers (mohawk-finishing.com).
Grain filler comes in a range of colours, from natural to dark brown. Choose a colour that compliments the colour of the wood you are using, or the final colour you want to achieve if you plan to stain the wood. Your surface should be sanded before applying the filler; I usually stop sanding when any milling marks or sanding scratches are fine enough that they don’t show, usually at 150 or 180 grit.
Step One – Seal Open Pores
This is an optional step, though one I always take. If you don’t apply a seal coat the filler will colour the wood in addition to filling the pores. If you apply a seal coat, the filler will only fill the pores, and not impart any colour to the wood. The seal coat is nothing more than a thinned coat of the finish that you apply to the wood. If you are using varnish or polyurethane, dilute varnish by 50% with mineral spirits, apply a thin coat and let it dry for several hours. Remember that varnishes do darken most woods. Alternately use a super blond shellac if you want to impart the least amount of colour to the wood. There is no need to sand the seal coat.
Step Two – Stain the Wood
If you plan to stain the wood, do it now. Two of the few woods that I stain are mahogany, as I think it looks so much more elegant with a brown mahogany stain and red oak, a great wood to work with, but I really dislike its bland, amber tone. I apply the stain as per the instructions on the container, and let it dry thoroughly. In a case like this I’ll also choose a dark grain filler. Optionally, you can mix the stain in with the filler, or add a tint, such as Mixol Universal Pigments to the filler and apply both at the same time.
Step Three – Apply the Filler
You’ll need an applicator (used paint brush, plastic trowel or stiff piece of cardboard) and some coarse scrap material – that flannel shirt with the monkey photo on it that Aunt Bertha sent you is a good choice – and a clean piece of cotton cloth. Begin by mixing the filler thoroughly. If you find it too thick you can add some mineral spirits to bring it to the consistency of pancake batter (that is, for an oil-based filler). Use the applicator to spread the filler across the surface of your work piece. Then use some scrap material to rub the filler into the wood pores. I use a figure eight motion, rubbing for a couple of minutes.
As you rub you’ll notice the filler beginning to thicken. This can happen pretty quickly, depending on how much mineral spirits you added to the filler. At this point rub the filler off against the grain. Then take a clean piece of cloth and do a final light clean up of the surface, rubbing with the grain. Allow the piece to dry overnight. For woods with deep pores, like walnut, you can apply a second coat of filler. Just repeat the same method outlined above. Once the piece is dry you can lightly sand with 320 grit sandpaper.
That’s it. You’re now ready to apply your chosen finish. For an ultra smooth, glossy finish try applying four or five coats of varnish, letting each coat dry in between. Allow the finish to cure for a week or so, and then wet sand with wet/dry paper or Abralon sanding pads from 800 to 2000 grit. Top off with a buffing compound, and then an ultra fine rubbing compound. For more information read Rubbing the Finish.