A very bossy article about using woodfiller on hardwood floors

Woodfiller does not permanently fill gaps

Notice the important word here: permanently. Woodfiller can temporarily fill anything, even the annoying gaps that run parallel to the long edges of your floor boards. But the size of those gaps changes all year long and the woodfiller in those gaps isn’t elastic enough to swell and shrink with the seasons and so will just get pummeled and eventually break up and look like kitty litter.

Kitty litter trapped forever in your flooring gaps. Open, honest gaps are far more attractive than dirty, poorly filled gaps.

My Floor Guy says I should always trowel-fill floors before sanding. What does that mean?

Trowel-filling is exactly what it sounds like: spreading large quantities of filler over the entire area of a floor, allowing it to dry hard, then sanding off everything that doesn’t fill a void.

In some cases (and in the hands of a professional) this procedure can have some benefits: on very new floors in climates where the boards don’t swell and shrink very much through the seasons, trowel-filling does help keep the finish contiguous.

But in older, already gappy floors in climates with season extremes (yes, like our store in Minnesota) the benefits of trowel-filling are short-lived. Within a year, the movement of the floor will break up the filler, leaving you with a finish which is no longer contiguous, plus a lot of loose fill between the boards.

In the second photo above, the filler must have look great when it was freshly done – it even took the color of the stain perfectly. But just a few years later, it looks like someone tried to fill the gaps with Nutella.

Woodfiller is not a substitute for board replacement

Wood has grain; woodfiller does not, so large areas of woodfiller look nothing like the wood that surrounds it. Even when it does take on the color of the stain or finish you use, it will still look like a big, undifferentiated blob on your floor.

When you use filler on nail holes and chipped board corners, it camouflages well in the grain pattern of the wood. But large areas will stick out like a black eye. This is especially important if you have hollows or depressions in your floor – these are better simply sanded clean and left alone. Hand-coat those spots if you must to keep the finish from getting too thick, but do not try to “level” a hardwood floor with filler.

The bigger the area you fill with woodfiller, the more it will shrink.

Larger voids also require that you apply the filler in stages (because it will shrink after each application). And the more applications, the longer it will take to dry before you sand it. If you are using so much filler that it takes more than an hour to dry hard enough to sand, you are probably using it inappropriately.

I get it – you don’t want me to put filler in those cracks. But my finish will run into those gaps and fill them nice and flush, right?

Again, no. Floor finish is designed to be quite runny so it will flow and seek its own level on your floor, and it will run right through your cracks and down to your subfloor. Glues and epoxies will do the same thing because the gaps in your floor have no bottom.

My hardware store has these cute little pots of Color Putty in dozens of colors – why can’t I just fill all my cracks with that?

This is the perfect juncture to explain that there are two different kinds of products you can use on a floor. Up until this point we’ve been talking about the kind that starts out soft and pliable so you can work it into all the holes, but it dries hard and must be sanded flush to the level of the floor, and to remove all the extra filler that went into the wood grain around the hole.

This kind of filler comes in a narrow range of colors and you choose the one that best matches the background color of your RAW wood, and it should then take on color (stain or finish) as your wood does. Color Putty and other soft putties are designed for wood that is already finished. These fillers are already pre-tinted in a much wider variety of colors than hardening fill. Because you simply press them into the hole and you’re done, you need to have a precise color match to the finished tone of your wood.

The problem with Color Putty is that it NEVER HARDENS!

It is designed this way – this allows you to putty it into the void in your floor, and then wipe away the excess without having to sand your nice finished floor. But Color Putty stays soft and, if applied in frequently walked-on areas, will eventually work itself out again. You can easily reapply it, but it is something you have to keep up with.

I’ve heard about filling my wood gaps with rope – why can’t I do that?

It’s true that rope won’t crack and fall out like filler, but it won’t look like wood either.

At best, it will reduce the amount of sand and debris that can collect in your floor cracks, but it won’t do much else. You can use rope as a way to fill up a big crack and then just top up with filler (so you don’t have to use as much filler), but the filler itself will still crack and fall out over time; there will just be less of it.

If your gap is truly wide enough to even consider filling it with rope AND the gaps don’t close up in summer, use cork instead. It’s sold in strips of different thicknesses, it’s easy to cut and you can use a cheap dental tool (Harbor Freight has good ones) to push the strip of cork into the gap.

You can even use a little paintbrush to apply finish to the cork when you’re done.

If you’re using cork on gaps, do it after you sand and finish the surrounding floor. Cork is also a great material to use where leaving an expansion gap might be unsightly, for example when wood is installed flush against a fixed obstacle like a fireplace or a sliding glass door.

Then what should you do with wide gaps, big holes and gouges?

First, try to fill them with actual WOOD; it doesn’t sink and it shrinks and swells with the wood surrounding it. If the problem area is too small to warrant removal of the board, but too big for fill, drill it out to a regular circle and fill it with a face-grain wood plug (or even several in a row) in the same species as your floor. These blend into the floor beautifully, and are a fraction of the work of replacing a whole board.

Slices from a dowel will work in a pinch, but they will absorb stain more deeply than the wood of your floorboards and will appear as a darker circle; face-grain plugs match the grain direction of the actual flooring and will blend and take finish much more seamlessly.

We sell the most darling, tapered face-grain plugs in maple, red oak, walnut and cherry in various sizes. For wide gaps, use a Dutchman – a tapered shim of wood that is pounded in to the gap, nipped off and sanded flush.

When you do choose to use filler, there are some products that we adore and use on the contracting side of our business and will be delighted to sell to you.

We adore this pre-mixed filler because it shrinks less than any other filler we have tried, which makes it a better choice if you insist on filling larger areas. It also takes the color of stain, Rubio and polyurethane better than other fillers. And even if it dries out, it easily reconstitutes with water.

We carry it in Red Oak, White Oak, Maple, and Ebony. Also can be tinted with universal colorants if you need to make a custom color.

Glitza Wood Flour Cement mixed with 80-grit wood dust that matches the wood of your floor.

Don’t use wood glue and dust to make filler – it always dries more yellow that you think – unsightly! The wood flour cement has two main strengths: the epoxy itself lasts forever, and you can mix up only as much as you need for each job, so there is no waste.

Second, you can mix up a different color for the spot you’re filling, just by mixing in a different color of dust. Because sometimes, a light spot in an oak board will actually look better with maple-toned fill.

Unfortunately, this is only available by the gallon and, because of its toxic nature, cannot be shipped.

$150 per gallon. Available only at our store in St. Paul, MN. Call 651-698-5888 to order.

Once you’ve decided to use woodfiller, use it responsibly! These tips will help:

• Fill after you’ve sanded through 36-grit. Don’t fill at the very beginning – you won’t see all the places that need fill. But don’t wait until the end of the job either because the filler that sticks in the wood grain around the void you’ve filled needs to be completely removed or it will look like a liitle halo around each filled spot, once the finish is applied.
• Match the filler to the color of the spot you are filling, not just to the specie of the whole floor. Maple floors have dark spots and oak floors have light sections, and all floors have at least one dark knot. Use your eyeballs, not the name of the filler to guide you. If you are filling knots, use an ebony fill, or use universal tints to darken a small amount of your red oak fill. But don’t assume that one color will fit all your filling needs.
• Give your filler time to dry before you sand it flush. We recommend applying filler right before stopping for lunch or before you leave for the day. As long as you aren’t trying to fill the Grand Canyon, it will be dry by the time you’re ready to start sanding again.