Choosing The Proper Fill For Gaps In Old Douglas Fir Toungue And Groove Flooring


Hi Joseph,

Here is my question again regarding filling the cracks in an old doug-fir floor:

I have very “rustic” quality doug-fir tx floors. There are a lot of gaps between the floor boards and numerous chips and gouges everywhere. Regardless of the distressed quality, I would like to fill the gaps, sand it down and apply a light stain to the wood. Unlike most people, I don’t mind a floor with lots of character. It’s what I’ve got and I’m willing to live/work with what I have. I’ve heard that you can color wood filler to match the final color of the floor once its been stained. I’m interested in using a non-toxic stain and have been advised to use the Livos brand Meldos Hard Oil, which is a linseed and orange oil stain.

Apparently the stain has a slight yellow tint to it that will darken the wood and bring out a more reddish hue. How can I tint the wood filler so it doesn’t stand out in stark contrast once I begin staining my floors?




Dear Kirk

There are many ways to fill a floor. You can certainly fill the gaps before sanding, and then when you finish the floor more of the oil or varnish will stay on the surface of the wood, rather than running down between the gaps.

So, I’ll go over all your choices. The cheapest and still the most effective fillers are simply made by mixing up fine (from a floor edger machine) maple or red oak sawdust and a lacquer based finish. It dries to a pleasant reddish color. This filler is quite hard, and sticks fairly well to the the gaps. It will survive the sanding, but in time it will crack out as the boards move seasonally. But this could take a couple of years to occur. It will accept the pigment in the oil finish and remain just a bit darker than the floor, so just apply the finish over the filled floor.

One better filler than this would be the same dust mixed with a good oil based polyurethane varnish. You will need to add some whiting (powdered chalk) to lighten this up. The varnish causes the filler to turn very dark, so the whiting will help, and you can also add some universal tints. This filler is generally troweled on just after all the sanding is done. But it takes a lot of skill to keep excess filler off the boards. Have someone right behind you neatly wiping up the excess that does occur. This will act as a good sealer for the wood, but only if you are using the same finish for the following 3 coats. (Oh, oil finishes cannot be used as a binder for a filler). The advantage to this poly blend is that it dries very hard, but stays in the gaps almost forever. I used it on my floors 22 years ago, and most of it is still there. Tough job though. This filler doesn’t accept much color once dry, so you have to get the color right in the mix of ingredients.

Next choice would be the rope trick. Again after the floor is sanded AND a coat of finish is applied, get various sizes of cotton or hemp rope. Pre-stain these with a regular wood stain, to match the final color of the wood. Stuff these ropes into the gaps, using the various sizes to fill the various widths of the gaps. You can even unravel a section of the rope to down size it and make it fit better. Don’t be tempted to glue the rope in place. This may cause the floor to “panelize” as it shrinks in the heating season, and pulls a bit of it’s glued neighbor off in the same direction. Instead apply the coats of poly to the floor, and the rope will just absorb it and dry and get stuck in place. The rope method doesn’t look nice, and you do have to keep the rope below the surface, or else your bare feet will feel it texture, on an otherwise smooth floor. But it’s easily replaced.

Next down the list of choices would be the water based fillers in the big 5 gallon tubs. There are many many makers of type of filler, and they range from very poor to mediocre. This stuff can be troweled in before and even just after the coarse sanding process. You can use a masonry trowel at first then later during the sanding stick to a 12″ drywall trowel. The drywall trowel applies the filler in a more controlled manor, but the filler may have to be thinned to make this work. Water thinning makes this stuff weak and crackable. It would be better, albeit expensive, to thin this stuff with a good water based finish. Here are a list of brands to try :

Dura Seal’s Trowelable Wood Filler at

Bona Kemi’s Pacific filler at

Berger-Seidle Aqua Choice Trowelable Filler at

Almost all the water based trowel fillers come pigmented already, so choose a color that is a just a tad bit darker than your final finished floor color. Oh, and speaking of finishes, the one you are about to use, is not very non-toxic, nor durable enough for floor use. I would try the Tried and True brand of varnish/oil if you HAD to use an oil finish. This product contain no solvent of any kind, check it out at


And that’s the key to any filler, it has to have some sort of varnish-like binder in it to make it hard, stick to the wood, and it keep flexible. The last quality is not really achieved except by the rope. I wouldn’t, in any case expect the filler to match the various color of the wood floor, and trying to stain by hand it with a wood stain would be one of those “forever jobs”.

Just one more note, I always use 4 coats of oil modified polyurethane on all my softwood floors. These floors will last longer and look better if finished in this matter. You can read the introduction to the use of this great finish at