How to Choose and Use Fillers for Wood Floors

by Hal Rusche of Heritage Hardwood Floors

Note from NH:  If you are purchasing and installing your own hardwood floors, this article should be of particular interest to you if you want the most "professional-looking installation.

All hardwood floors get filled during installation, sanding, finishing and use. If you take a close look at any prefinished wood flooring, right out of the box you will see that it has been filled. There are many different types of wood floor fillers; some are designed for general application where others are more specific. The purpose of this article is to remove some of the confusion about fillers and to help people choose the right one.

Latex Wood Floor Filler

The most common and widely used wood floor filler is latex. Latex filler is water based, stainable similar to soft wood, easy to apply and sand. All manufacturers produce their fillers is several popular colors, many provide colored fillers to match most wood species including imported woods. Almost all fillers come premixed and ready to use, a few can be purchased in a powder form that need to be mixed by the user. If your filler is not properly matching the color of your wood floor, you can mix different colors together or add something else to affect the color.

The way that the color was originally produced was by using universal colorant; therefore you can tweak the color by adding more or different universal colorants. A few things that I have used to alter the color of wood filler are; latex paint, chocolate syrup and food coloring. Some wood flooring contractors mix oil based stain into their latex wood floor filler, apparently it works although it doesn't sound like a good idea to me, possibly water base stain would be a more compatible colorant.

Latex filler is made of clay, calcium carbonate, wood fiber, universal colorant, water and some kind of binder. One natural binder that I can think of is egg yolk, without a binder the filler will crumble and fall apart after the water evaporates.

Latex wood floor filler is not an adhesive, it will not glue the floor boards together and can not be used to glue a chip or sliver of wood into a floor. This filler can be thinned by adding more water to it, if your want to skim the entire floor, full trowel filling, to seal the grain during the sanding process more water will be needed, but realize that the more water is added the greater the amount of filler shrinkage you will get. Large voids require several applications.

When latex filler is wet, it is close to the color that it will be after it is sealed with polyurethane, varnish or wax. This filler needs to be under a coating of something, if not it will dissolve and leave a latex haze when ever the floor is cleaned with water based cleaners.

There is one trick that I have used colored latex filler for that is time consuming and labor intensive but it works. When you work for home builders you will always be asked to do things that are not right. You can protest but in the end if you don't comply they will find someone else who will.

The situation is that you have a floor that has been finished and sitting in a house for a long while that has no humidifier. The floor has gaps between most of the flooring boards because the floor and the house are overly dry and the floor boards have become dimensionally smaller. To properly swell the flooring back to the original sizes would require the installation of a whole house humidifier and take at least three months.

To fix the problem I took several different colors of floor filler, using them all and a putty knife, I filled the gaps in areas of about nine square feet of flooring at a time. After filling each area I washed the excess filler off of the floor using wet rags. I washed it again to remove most but not all of the latex residue. When I was finished I would look at the areas and if I could see the filled gaps too easily I worked a different color of filler into the area to change the color of the gaps to be less noticeable. I did this to the entire hardwood floor area. This process took so long that by the time that I was finished filling the floor, most of the filler had dried. Then, starting where the filler is the driest, I took a used 180 grit abrasive screen under a heavy floor buffer and screened, vacuumed and coated the floor with oil based polyurethane. Although not perfect, the improvement to the floor was huge.

Prefinished Wood Floor Filler

Prefinished wood floor filler is not so widely used because it is a fairly new product, expensive, dries out in the jar and gets ruined if allowed to freeze. This filler is only used on floors that are already finished, and prefinished hardwood floors. As always, it's best to work with several colors at a time, I suggest one light one dark and one warm color compared to the color of the floor. The different colors can be mixed together, trying to match the color of the grain. Nail holes should be made to match the color around them.

This filler has the same ingredients as latex wood floor filler except the water has been replaced with satin water based polyurethane. Therefore this filler stays almost the same color after it dries as it is when it's wet. The nice attributes of this filler is that after if hardens, it becomes water proof, therefore it needs no top coat and it has a very low shrink rate. If you're filling a wood floor between coats of water based polyurethane then this is the filler to use. If you're filling an already finished floor or a prefinished floor with this product always, wipe the filler and area off with a damp rag or a shinny spot will appear around the spots that were filled.

When using this filler, it helps to carry small amounts of this filler around in a damp rag, the damp rag will keep the filler from drying out while using it. This stuff will dry in your hand and stick to your skin. You need to be careful not drop crumbs of this filler onto the floor, if you step on them they will stick.

Wood Putty, Painters Putty

Wood putty is very common and widely used wood floor filler. Painters use it to fill nail holes between coats of paint and on stained wood work. Flooring contractors use it primarily between coats of oil based polyurethane. You can use it on a floor that is already finished without coating over it, the color stays fairly true but since it dries extremely slow, lighter colors will collect a lot of dirt and end up dark. This filler is made of calcium carbonate, boiled linseed oil and universal colorant.

We used to make our own wood putty on the job site, calcium carbonate and linseed oil are cheap, but over time it proved easer to buy wood putty already made. This product is very similar to window glazing compound, years ago wood flooring contractors would mix universal colorant into window glazing compound and use it as a wood floor filler. If your filling a floor between coats of oil based polyurethane, then this is the filler of choice. Never use this filler under water based polyurethane. If you're not coating over this filler with polyurethane, you will need to wash off the oil smudges that will appear on the floor around the filled areas.

Lacquer-Based Wood Filler

Lacquer based wood floor filler is a mixture of lacquer and very fine wood dust. For best results, use fine edger dust from the floor that is being sanded, mix it with lacquer to form a putty, and fill the floor as needed. This filler is fast drying, gives off a lot of strong fumes and is flammable.

Epoxy Floor Filler

I don't know how this stuff is made. It is a two part system, you mix the parts together and a chemical reaction causes the material to harden in five to twelve minutes. The only thing that this filler is used for is to fill open knots is rustic floors, before the sanding process. This material can run out the bottom of a knot, so a couple applications may be necessary. Usually a black or dark brown pigment is added to match the color of the knots.

Bondo (a.k.a. auto body filler)

Bondo is a putty made of polyester resin and cabosil, an inert filler, catalyzed by an organic peroxide. Bondo is not really a wood floor filler, but years ago I did use it on a few floors and it worked well. Bondo will not accept stain but if you color it to match the wood, it will work fine on a natural floor.

Professionally speaking,

Hal Rusche 2nd,  President, Heritage Hardwood Floors, Inc.