Successful gap filling on wooden floors is one of the skilled parts of floor finishing – a lot of floor sanding companies can’t or won’t do this. The most successful way despite taking longer is, (for gaps less than 4mm or so), to use a mixture of polyvinyl resin and the finest grade sawdust from the floor that is being sanded, (we use a wood floor resin specifically for this job). The mixed resin is spread accross the wooden floor at a specific time of sanding the floor and when dried, is sanded off leaving the remaining residual filler held within the gaps. For floor gaps larger than 4mm, slivers of wood (of the same type) are glued on both sides and carefully secured in,- if it is a pine floor, we use reclaimed pine slivers so they blend in and oak slivers for oak floors and so on.
Please note that, when choosing any of these methods, this does not create a ‘perfect floor’. There could be some degree of compromise on the final finish, as both floor filling and slivers filleting, do require adding third party separate pieces and/or filling with an artificial resin filler. Wood floors will naturally continue to expand and contract with temperature and age and, whilst the resin product is designed to expand and contract to a point, higher levels of movement can result in gaps re-appearing, (albeit usually to a lesser degree).