Floor varnishing fails most often not because of the varnish product itself but because of inadequate preparation. The varnish film sits on top of whatever surface it is applied to; it cannot hide machine marks, colour stains, gaps in the floor or incorrect moisture levels. Investing time in proper preparation before opening the first tin of varnish determines whether the end result looks professional or problematic.
Checking the Subfloor and Board Condition
Before sanding, walk the floor and identify any boards that are loose, squeaking, split, or significantly raised above their neighbours. Loose boards should be refixed before sanding. A loose board under a running sander will move during the sanding pass, causing uneven stock removal. Squeaking boards on timber subfloors can often be fixed by screwing down through the board into the joist below; countersink the screw and fill the hole with a colour-matched filler before the final sanding pass.
Boards with significant splits or damage may need to be replaced rather than sanded over. A badly split board will remain visible through the varnish regardless of how it is sanded. Replacement boards should be acclimatised to the room conditions for at least 48 hours before fitting.
Gap Filling
Gaps between boards are a common feature of older softwood floors and some solid hardwood floors. Whether to fill them depends on their size and the look you want. Very narrow gaps (under 1mm) can generally be filled with a flexible floor filler that is sanded back flush before varnishing. Wider gaps (over 2mm to 3mm) often need a resin filler or carefully fitted wood slivers rather than flexible filler, which may shrink back below the surface over time.
The most practical approach to large gaps in older floors is Bona Mix and Fill or a similar resin filler: sanding dust from the floor is mixed into the resin to produce a colour-matched filler that sets hard, can be sanded flush, and does not shrink. Apply after the first sanding pass so the filler contains dust that matches the freshly sanded floor colour.
Sanding the Floor
Sanding removes the existing finish (if any) and produces a clean, level surface for the varnish to bond to. Use a drum sander for the main field and an edge sander for perimeters. The standard grit sequence for a residential floor is 40-grit, 60-grit and 80-grit. On new or previously unfinished floors, start at 60-grit if the surface is already reasonably clean and level.
After the final sanding pass, inspect the floor under raking light. Any remaining marks, ridges or uneven areas will be visible under this examination and should be addressed before varnishing. A pole sander with 100-grit paper is the right tool for localised attention after the main machine sanding.
Moisture Testing
Wood varnish applied to a floor with excessive moisture content will fail. The moisture in the wood migrates through the new varnish film as it tries to equalise, causing bubbling, clouding or adhesion failure. Check the moisture content of the boards using a pin-type or capacitance moisture meter before varnishing. Most wood floors should be below 10 to 12 per cent moisture content before varnishing. Concrete subfloors beneath timber floors should be below 75 per cent relative humidity at the surface.
- Fix loose and squeaking boards before sanding
- Replace damaged boards that cannot be successfully sanded
- Fill gaps after the first sanding pass using colour-matched resin filler
- Sand through 40, 60, 80 grit sequence
- Inspect under raking light after final sand and address any remaining marks
- Check moisture content before varnishing
- Allow new concrete subfloors to dry fully before varnishing boards above
Preparation is slow, dusty work compared to the relatively quick and satisfying process of applying the varnish itself. The temptation to skip or rush preparation stages is understandable, but preparation quality determines the final result entirely. A well-prepared floor with even mediocre varnish looks better than a poorly prepared floor with excellent varnish.
