Using Wood Filler to Repair Wood Floors

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No matter how careful we are, nicks, dents, and mistakes happen more than we would like to admit in our industry, and it’s common practice to fill in nail holes and minor gaps with close-matching wood filler.

When extreme repairs are required on severely-damaged areas, these situations often require board replacement. Using wood filler on large jobs is usually not recommended.

To increase your success rate with flooring installation and repair, it’s always necessary to research the characteristics of your wood species to determine stability before repair, and more importantly, before installing wood flooring.

When extra time is taken to measure moisture content at the time of installation and the expected “in use” changes are also considered, potential problems can be avoided. Flooring can be expected to perform well for years to come without any major repair issues.

The NWFA suggests normal living conditions in the home should have relative humidity levels between 30% and 50%.

Even though it’s normal for homes to become dry during the winter months, causing wood flooring to shrink, flooring that has been installed and maintained properly should remain stable and exhibit normal gaps.

If you find it necessary to repair gaps that aren’t normal seasonal gaps, the best time to initiate repair for the majority of locations would be outside of the extreme cold and winter months; in April or October.

During the summer months, normal seasonal gaps will usually close up and not require repair. If wood filler is applied prematurely between the strips on dry flooring, the material will be pushed up through the gaps when the floor picks up moisture and expands.

When we met up with Cape Ann’s prestigious hardwood floor refinishing and installation specialist Chris Pester from CFC Hardwood Floors, Inc. (the same guy who was invited to sand the deck of the Boston Tea Party ship, “The Eleanor”), we asked him: What is the most common problem that causes gapping in wood flooring? He explained:

“I live in New England where we get very dry winters. We do have high-end customers who keep their homes humidified in the winter and dehumidified in the summer. They’re climate-controlled to be kept between 30% and 50% relative humidity year-round as suggested by the NWFA and the Indoor Air Environmental Association.”

“If you can keep your house between 30% and 50% relative humidity year-round and between 60 to 80 degrees year-round, you can expect to see minor seasonal movement and your filler to perform well under those conditions.”

“For the rest of us who cannot keep our house between 30% and 50% relative humidity, we’re going to expect our floors to move a certain amount. If they move more than 1/16th of an inch, we can’t expect our filler to perform well. We’re going to expect to see minor gapping in the winter, and a tight floor in the summer.”

When we asked Chris how he measures the moisture content of flooring, and what instrument he would recommend, he responded:

“I have a Wagner wood moisture meter and I love it! I have two moisture meters…I have a penetrating meter and I have a non-penetrating meter. I love the non-penetrating meter because people don’t like it too much when you stick holes in their floor.”

“Every good floor installation guy should have a wood moisture meter or two in his toolbox and he should know how to use it. He should know that different wood species have a different specific gravity and the wood moisture meter needs to be calibrated correctly to the species.”

“Unfortunately, most of my competition doesn’t own wood moisture meters. There’s an absolutely ridiculous guideline where everyone says, ‘Just bring in your wood a week before and it will be fine.’”

“If you bring the wood in the week before and the guy is still painting or plastering, well, then you’ve just absorbed a huge amount of moisture from the paint that goes right into the wood. The wood has expanded and then it’s going to contract. You’ll end up with more gapping than you otherwise should.”

“If contractors took the NWFA courses and learned their trade better, we’d have fewer problems in the industry. We’d have less issues that people think are problems because there’s so much misinformation out there.”

“In my experience, to bring in flooring at the right time, make sure everything is acclimated correctly and keep your eye on it in the future; you’ll never have a problem with a hardwood floor.”

When we asked Chris to recommend the best wood filler for hardwood floors, Chris replied:

“You’ll find one of the problems with wood filler here in the Northeast is this…we face massive moisture swings throughout the year, and we see quite a lot of seasonal movement, so of course when that floor moves, so does the filler. The filler often gets cracked, chipped, and pops out and you’ll end up vacuuming it out for the next year or two.”

How to Use Wood Filler and What to Use:

What you want to use is something elastomeric, meaning that it can shift, shrink and expand just a little bit with seasonal movement. That type of filler will tend to last longer.

There is a wood flour cement that is more expensive and time-consuming, but you’ll get a better color match. One example is made by Glitsa.

Hardwood FloorWhen asked about matching floor color with wood filler, Chris said:

“There are certain species of wood…for example, fir and pine have a lot of color variation in the boards. If you sand the whole floor (only use fine dust 80 or above otherwise it’s too coarse and it doesn’t stay in), use the dust gathered from the entire floor. If you put the filler into a light board or a dark board, you’re crossing the range. With certain species, filling can be more problematic.”

Problems That Can Occur When Trying to Match Wood Floor Color:

The pre-mixed wood fillers that you can buy from companies like WOODWISE or Timbermate sell many different colors, including Brazilian cherry filler. When you open a pot of Brazilian cherry filler, the color is pink. When it darkens over the first year or two and becomes a beautiful, rich, dark, reddish-brown as it usually does, you’ll find the filler won’t match the floor anymore.

Other Problems Chris Faces with Wood Filler:

“When we’re sanding a floor that’s a couple of hundred square feet and we only have one day to get it sanded and sealed, I like to get three coats of filler into some of those areas but I just don’t have time for it to dry. So when the floor is vibrating with the drum-sander, and the filler doesn’t have time to properly stick, you won’t be able to achieve a great result.

“I always ask the customer, what’s the purpose here? Should we take our time and charge a little bit more money and get a better result? Or are you looking for a quick fix? If it’s a quick fix, then I’m going to get one or two coats of filler in there and do my best.

“If we’ve got a big job, what I’ll do is this: I’ll send my guy into a closet and have him sand it the whole way down. Then he’ll collect clean, fine sawdust to use for the rest of the floor. If we know we’re there for a week, we can get several coats on and get proper dry times in between. That way we’ll have more success with the filler.”

When asked if Chris had any horror stories of wood filler experiences gone bad, he had this to say:

“I get called in to go and inspect floors all the time. In the old days, guys used to put in all sorts of nasty things. I’ve seen leveling compound and such atrocities used as filler, I’ve even seen mastic, I’ve seen yellow wood glue…I’ve seen horrible things put in.

“The other day I was called to look at a birch floor. The guy had put yellow wood glue in the gaps…I ended up having to chip it all out so I could get filler in over the top of it.”

How Long Does Wood Filler Take to Dry?

“Preferably overnight, or at least 6 hours minimum. When I’m in a hurry, I have filled and refilled with water-based filler within an hour.

“Also, if you have big holes that need filling, never try to fill all at once. Fill a small quantity and let it dry. Fill it again and let it dry. Always allow a 6-hour minimum dry time in between layers if you can.”

Types of Wood Filler:

There are two different types of wood filler products: water-based and solvent-based. Solvent-based fillers tend to dry faster than water-based fillers, and water-based fillers are easier to clean up on the job site.

How to Prepare Wood Surfaces for Wood Filler:

If the area you are working on is finished and coated, make sure you sand off and remove all the varnish. When you have removed the varnish, continue to sand the wood in the direction of the grain with 120-grit sandpaper to ensure all of the finish is removed and the surface area is smooth.

Vacuum and remove sawdust from the area to ensure the wood filler will adhere properly to the work surface.

How to Fix Cracks in Hardwood Flooring:

Once the damaged area is clean, apply wood filler by pressing firmly with a putty knife. When the area is dry, sand the filler until it’s smooth and flush with the surface. A sanding block will help you achieve a good result with a flat surface.

How to Fix Holes in Wood Flooring:

Always use shallow applications when filling deep holes. This will help prevent cracking and shrinking. Before you apply the next layer, ensure the layer is completely dry. If you attempt to sand the area and the filler clogs your sandpaper, the area is not dry.

Refinishing a Repaired Area on Wood Flooring:

The NWFA suggests using products from the same manufacturer to help ensure product compatibility and proper adhesion. If you are going to apply a stain or finish product from a different manufacturer, you are advised to contact your manufacturer to check product compatibility.

Timbermate Water-Based Wood Filler:

Timbermate water-based filler is no stranger to the market. It’s a product that many contractors recommend.

Timbermate wood filler can be used on all types of flooring when you find it necessary to fill cracks, gaps, and nail holes. The product is recommended for all species because the company claims it doesn’t shrink, sink, crack, or fall out.

The product will also take all types of stain and colorants that are water-based, solvent-based, oil-based and acid catalyzed lacquers. You can apply stain before or after applying Timbermate.

When matching colors with Timbermate, it’s recommended that you add floor color to the putty and match the color while it’s wet. The product will dry lighter, and return back to its original color when coated with a clear finish.

How to Apply Timbermate:

Timbermate can be applied into cracks, holes, or blemishes on the wood with a putty knife. Deep holes should be filled in layers of 6mm, allowing each layer to completely dry between applications. When sanding to prepare for stain or varnish, it’s best to use a fine sandpaper of 120-150 grit.

The experienced refinisher has to draw from his experience to use the right filler for the right job. As Chris Pester advises, “The motto is always: Test. Test. Test. If you’re not sure, do a test on a board in your workshop the night before installation.”

There are so many variables with wood filler and so much ambiguity…it’s something you learn over a long time. There aren’t a lot of hard fast rules.

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Larry Loffer

Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.

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