I am trying to get some ideas on what I can use to fill gaps between the floorboards on my heart pine floor. Circa 1929, in good shape except the stuff ( dried wax& dirt ) between the boards. Some gaps as wide as 1/4'. Are there fillers that are flexible enough to move a little as the boards move? Also, I plan on staining, so a filler that will accept an oil stain. There seem to be products that get troweled on then mostly removed and then sanded back to wood. I am also removing paint on pine trim and will be using the same stain on the trim. I need a filler that can bridge vertical gaps while not running and be stained.
Any help would be appreciated.
Thanks

Zar wood patch may be what you are looking for. The failure of wood patch filling gaps between boards can be helped by fastening strips of wood, even just one-half inch plywood, fastened under the flooring between the floor joists with construction adhesive and screws or short staples to hold the strips in place while the glue dries will also help to firm up the floor.
Filler will pop out when the flooring deflects with traffic, since without any sub-floor the movement over time will weaken the bond holding the filler in place.
Vacuum the gaps after using a slotted screw driver to help clean out any loose dirt.
Press the filler in very firmly. You may need two applications of filler to fill deep gaps.
The oak colored wood patch is what you want to use. It will absorb stain to help blend with the flooring. Nail any loose butt-joints with finish nails, set nails and fill before any sanding.