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Berger-Seidle AquaSeal PAKStop 4.5 kg canister with a blue and white label, photographed on a plain white background
October 6, 2026 By Bill Lazzio

Filling gaps in floorboards: when to fill, when to leave them, and how

Ready to buy? EcoGrit stocks the products in this guide and ships Australia-wide. Shop the wood floor filler range.

You have sanded back an old floor and the joints now show as dark lines, some hairline, some wide enough to catch a fingernail. Filling gaps in floorboards looks like the obvious next step. Often it is, but not every gap should be filled, and the ones that should need the right product at the right point in the sanding sequence.

Why gaps open between floorboards

Timber is hygroscopic and floor coatings are permeable, so boards keep taking up and giving back moisture. In dry weather they shrink and gaps appear. In humid weather they swell and the gaps get fewer and smaller. A finish may slow how quickly a floor reacts, but it will not stop the movement.

The Australian Timber Flooring Association (ATFA) says an unrestrained 80 mm board in many hardwood species shrinks or swells by about 0.25 mm for each 1% change in moisture content. Moisture content when laid, local climate, indoor conditions, board width and fixing method all affect whether a floor gaps. ATFA also warns that fireplaces, appliance vents, uncurtained windows and houses closed up for long periods can cause greater gapping.

When to leave a gap unfilled

WoodSolutions says gapping across a timber floor should be expected, and ATFA notes that some floors show gaps only in the drier months. Leave the filler in the tin when:

  • The floor is new and the gaps run along the board edges. ATFA says edge filling is generally not recommended on new floors, except flood-filled parquetry and some direct adhesive fixed floors.
  • It is the expansion gap at the walls. WoodSolutions calls for at least 10 mm between a fitted floor and the walls, and ATFA accepts that this gap stays visible for the life of the floor.
  • Heat or moisture is still driving the movement. Fix that first. Berger-Seidle says PAK-Stop is not a vapour barrier against rising moisture.
  • You cannot get the joints clean. The data sheets for all four fillers below call for wax-free joints.

ATFA does expect punched nail holes, and end gaps on solid tongue and groove floors, to be filled.

Which filler suits which gap

Gap width and movement decide it. Here is the short version of our wood floor filler collection:

Gap Filler Mixed with Ready to sand
Most gaps up to 2 mm, across the whole floor AquaSeal Pafuki Fine sanding dust from the same floor Usually 30 to 45 minutes
Small joints, when you want a pre-coloured filler AquaSeal PafukiPowder Water only About 20 to 30 minutes
Large gaps and joints that move AquaSeal PAK-Stop About 30% PafukiPowder At least 16 hours
Small gaps and small areas, from a cartridge AquaSeal Pafudima FlexFill 310 ml Nothing, it comes ready to use When fully dry

Prepare the joints before you fill

  1. Fill after the coarse cuts. Pafuki uses 80 grit dust and PAK-Stop goes onto a floor sanded to 60, 80 or 100 grit, so fill there, then do your fine grits. Our floor sanding grit guide sets out the sequence.
  2. Clean out the joints. Vacuum them and remove old filler and wax.
  3. Close the wide ones first. Berger-Seidle says joints wider than 2 mm should be filled with pieces of wood, PAK-Stop or Pafudima FlexFill before a Pafuki coat.

How to fill fine gaps in floorboards with Pafuki

AquaSeal Pafuki is a water-based filler liquid for most gaps up to 2 mm. Mixed with the floor's own sanding dust, it takes on the colour of the timber.

  1. Collect the right dust: fine 80 grit dust from a multi-disc machine, taken from the floor you are filling and free of wax and dirt. Berger-Seidle says dust from belt sanding along the boards is unsuitable.
  2. Mix in a container to avoid spots on the timber, into a paste you can trowel. Too little dust and the filler sinks into the joints as it dries. Too much and it may not grip, so the next sanding can tear it out.
  3. Trowel the whole floor quickly with a stainless steel trowel, packing the joints. One coat covers roughly 20 m² per litre, and less on wide joints.
  4. Let it dry. It is usually ready to sand in 30 to 45 minutes in normal temperatures. Large joints can take longer.
  5. Fine sand until no filler is left on the face of the boards.

Prefer a set colour? AquaSeal PafukiPowder Color is pre-coloured and mixes with water only, never sanding dust. Berger-Seidle gives 5 kg of powder to 3.5 L of water for about 5.5 L of filler. Too much water and it settles in the gaps; too much powder and it can tear out. Pot life is about 45 minutes, and with small joints it is sandable after about 20 to 30 minutes at 23°C and 50% relative humidity. We stock seven colours. If you use one colour, ATFA advises matching the darker tone in the boards.

How to fill wide or moving gaps with PAK-Stop

AquaSeal PAK-Stop is a clear, high-elastic trowel filler that allows for expansion. PafukiPowder in a matching colour gives it colour and hardness, and our PAK-Stop and Pafuki Powder kit comes pre-measured at the 30% mix.

  1. Mix just before use: about 30% PafukiPowder, for example 1.35 kg to 4.5 kg of PAK-Stop, stirred mechanically until the paste is even. Kept closed at room temperature, a correctly mixed batch stays stable overnight.
  2. Trowel the whole sanded floor with a stainless steel trowel, leaving no residue. Expect about 10 m² per litre, and considerably less on big joints.
  3. Top up big joints. PAK-Stop shrinks in very large joints as it dries. That is normal: give them a second coat after an overnight dry.
  4. Wait at least 16 hours before sanding. Cold, high humidity, poor ventilation and thick coats can slow it considerably, and very oily species can take up to 36 hours.
  5. Sand back to bare timber at 60 or 80 grit, check every joint is sealed, repeat where it is not, then do your fine grits.

PAK-Stop takes more time and abrasive to sand than a standard filler, and it stays slightly sticky after drying, so Berger-Seidle says to mask the belt sander's wheels. On an old floor still bedded in its original adhesive, water-based coats swell the timber, which could damage that bed, so Berger-Seidle recommends one roller coat per day.

Small gaps and small areas: Pafudima FlexFill 310 ml

AquaSeal Pafudima FlexFill 310 ml is an acrylic, silicone-free joint filler in a cartridge, made for small gaps and small areas. It is sandable, can also be used as a glue, and we stock it in 12 colours.

  • Gun it into joints that are clean, dry and vacuumed, with no oil or wax, and mask the joint edges with smooth tape.
  • In wider joints, Berger-Seidle specifies a backing cord so it bonds to the two board edges only. Bonded to the subfloor as well, the joint is less durable.
  • It shrinks as it dries, so a joint may need a second sand. The final colour shows once it has fully hardened.
  • It also stays slightly sticky after drying overnight, so tape the sander wheels here too.

Sealing and coating over filled joints

All of these fillers can be overcoated with AquaSeal products once fully dry. Seal with one coat, not cut back: FlexPrimer for common timbers such as oak and pine, or ExoBloc or UniQuick for tannin-rich timber. See our sealers and primers, and our AquaSeal 2K PU guide for a full prime and topcoat procedure.

When gaps come back after the job

Filling does not stop a floor moving, so some joints may open again in a dry spell. Berger-Seidle's PAK-Stop data sheet allows for this: new joints from shrinkage can be sealed later with Pafudima FlexFill, which can be used on finished floors too. To reduce the swings, shade boards from direct sun and keep heater and appliance vents from blowing across them.

Not sure which filler suits your floor? Call our sales team on 0403 964 703, or technical support on 0401 270 818 for application advice.

Sources

Products in this guide

All available from EcoGrit, shipped Australia-wide. Trade pricing available for flooring professionals.

Timber Flooring Specialist

Bill Lazzio is a timber flooring specialist with over 30 years of hands-on experience in floor sanding, coating, and restoration across residential and commercial projects in Australia. Bill works with EcoGrit and leading European manufacturers to bring professional-grade, low-VOC products tailored to Australian conditions.

Filed in: berger-seidle, diy, sanding-prep, technical, trade-pro
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