To fill a void under a damaged wood floor, Club member Wendell Upright of Idaho Springs, Colorado, refilled an empty caulk tube with sand-thickened two-part epoxy so he could inject it through holes in the floorboards. Refilling a caulk tube also works well for injecting other one-shot mixes that set quickly, such as tuckpointing mortar (use a quart tube and gun for greater capacity).
A plastic tube that was filled with a latex product works best because it cleans up easily. Cut the tube short enough that you can get a knife under the edge of the plastic plunger; then grab the edge with needlenose pliers and pull it out. If you want to reuse the tube, glue the plunger to the caulking gun’s plunger. (You may want to dedicate an inexpensive gun for this purpose.) Depending on the mix, you may need to tape the end of the nozzle to keep it from running out while you fill the tube.