Mastic Fillers for Wooden Floors

Over time, even the most beautiful wooden floors can develop gaps between planks. This happens naturally as timber expands and contracts with changes in temperature and humidity. While small gaps might seem harmless, they can collect dirt, spoil the look of your floor, and even cause creaking. The good news? Mastic fillers offer a simple and flexible way to close those gaps and restore a neat, seamless surface.

There are many different fillers available, from traditional resin-and-sawdust mixes to modern flexible sealants. In this guide, we’ll take a closer look at some of the most popular mastic fillers for wooden floors, their pros and cons, and how to choose the right one for your home.

What Are Mastic Fillers?

Mastic fillers are flexible sealing compounds designed to fill small to medium gaps between wooden floorboards. Unlike rigid fillers, which can crack when the wood moves, mastic fillers expand and contract slightly, allowing the floor to “breathe” naturally. This makes them especially useful in older floors without tongues and grooves, or in rooms where humidity and temperature often fluctuate.

Most mastic fillers are water-based or solvent-based and come ready to use in cartridges or tubes. They are usually available in a variety of colours to match different wood tones, ensuring a more discreet and professional finish.

In short, mastic fillers are the practical choice for long-term gap filling—flexible enough to move with your floor, yet strong enough to keep dust and dirt out of the gaps.

Popular Mastic Fillers for Wooden Floors

Bona Gap Master

Bona Gap Master filler When it comes to professional wood floor restoration, Bona Gap Master is one of the most trusted names. This silicone-free, flexible filler is specially designed for wooden floors and comes in an impressive 14 different colours, making it easy to find a close match for your floor.

The filler has a 7% expansion rate, which means it moves with the wood as it naturally expands and contracts. It’s also completely odour-free, making it pleasant to work with indoors. Drying time is around 18–24 hours before sanding can take place, though full hardness is only achieved after 2–3 days.

Best used for: finishing touches, small cracks, and filling around skirting boards and beading. It is not suitable for very large gaps or end-grain floors.

Pros: 14 colour options, flexible, odour-free, professional finish.
Cons: More expensive than standard fillers, longer drying time, limited to smaller gaps.

Osmo Interior Gap Sealer

For homeowners who prefer an eco-friendly option, the Osmo Interior Gap Sealer is a strong contender. It’s a water-based, flexible filler designed for use on wooden floors, skirting boards, and other interior joints.

Osmo’s formula is low in VOCs, making it safe to use indoors and better for the environment. It’s easy to apply directly from the cartridge and can be sanded and overcoated once dry. Unlike rigid fillers, it maintains flexibility, which helps prevent cracking as the wood moves.

Best used for: general interior gap filling where sustainability and safety are priorities. Works well in homes with children and pets thanks to its non-toxic formula.

Pros: Eco-friendly, water-based, flexible, low VOCs, safe for families.
Cons: Limited colour range compared to Bona, not ideal for very wide gaps.

Junckers Gap Filler

As part of the Junckers flooring system, the Junckers Gap Filler is designed to work seamlessly with their solid hardwood floors. It’s a water-based filler paste that is typically mixed with fine sanding dust to create a natural colour match.

This filler is easy to apply and dries quickly, making it a practical choice during a professional floor sanding project. Once set, it can be sanded and refinished along with the rest of the floor, ensuring a uniform appearance.

Best used for: Junckers hardwood floors and professional sanding jobs where colour consistency and compatibility with the Junckers system are key.

Pros: Perfect match for Junckers floors, easy to use, dries quickly, blends well with sanding dust.
Cons: Works best with Junckers floors only, not as flexible as mastic-based fillers, and limited use outside sanding projects.

SikaFlex Wood Floor Sealant

If flexibility is your top priority, the SikaFlex Wood Floor Sealant is a strong choice. Known for its construction-grade durability, this product is designed to cope with significant movement while maintaining a tight seal. It’s more elastic than traditional fillers, which makes it highly resistant to cracking or shrinking over time.

SikaFlex is available in several wood-tone shades, and its high bonding strength means it can handle heavy-duty use. However, it is generally overkill for small household repairs and is more popular with contractors and professionals tackling larger restoration projects.

Best used for: high-traffic areas or floors with a lot of natural movement where maximum flexibility is needed.

Pros: Extremely flexible, highly durable, strong bond, ideal for demanding environments.
Cons: Harder to work with than lighter fillers, may be more than needed for minor home repairs.

How to Choose the Right Mastic Filler

With so many options available, choosing the right mastic filler comes down to your floor type, the size of the gaps, and your personal priorities. Here are a few things to keep in mind:

Colour match: A seamless finish depends on choosing a filler that blends with your floor. Brands like Bona offer multiple shades, while others may have fewer options.
Flexibility: If your floor tends to move a lot, go for a filler with strong expansion properties, such as SikaFlex.
Eco-friendliness: For a safer, greener choice—especially in family homes—Osmo’s low-VOC formula is a great option.
System compatibility: If you have Junckers floors, using their filler ensures a perfect colour and performance match.
Project timeline: Consider drying time. Bona, for example, requires at least 18 hours before sanding, while other products may set faster.
Budget: Premium fillers offer better flexibility and finish, but they do come at a higher price compared to standard alternatives.

Ultimately, the “best” mastic filler is the one that balances your floor’s needs, your budget, and your expectations. For high-quality restoration, professionals often combine fillers with other repair methods to achieve the most durable and visually pleasing results.

Limitations of Mastic Fillers

While mastic fillers are versatile and effective, it’s important to understand their limitations so expectations stay realistic:

Not for very large gaps: Mastic fillers work best on small to medium gaps. Wide openings usually need wood slivers or other structural solutions.
Natural movement continues: Wood will always expand and contract with changes in humidity. Fillers can flex, but they can’t eliminate this movement.
Reapplication may be needed: Over time, especially in high-traffic areas, fillers can loosen or wear down and may require topping up.
Slower drying times: Some products, like Bona Gap Master, need up to 24 hours before sanding and several days to fully harden, which can extend restoration projects.

In short, mastic fillers are a great solution for improving appearance and sealing gaps, but they are not a permanent fix for all floors. Combining fillers with proper floor fitting, secure boards, and regular maintenance is the key to long-term results.

Professional Use of Mastic Fillers

While DIY gap filling is possible, the difference between a quick fix and a lasting solution often comes down to professional application. Flooring specialists know which product works best for each situation, how to prepare the surface properly, and how to combine fillers with sanding and finishing for seamless results.

At FlooringFirst!, we use premium products such as Bona Gap Master and other trusted fillers as part of our floor sanding and restoration services. This ensures gaps are treated carefully, your floor’s movement is taken into account, and the final finish looks smooth and natural.

For expert advice or to book a free site visit, call us today and let our team bring your wooden floors back to life.