I like to mix my own wood filler using glue and sawdust. To ensure I always have sawdust on hand from the right species of wood, I store sawdust in labeled, resealable bags. Using scrap wood, I drill a series of larger holes with a Forstner bit, (stop short of drilling all the way through, though). Then, using a sandpaper grit of 120 or coarser, I sand the piece until the holes are filled. If you use an electric sander to capture the sawdust, be sure to clean the dust bag before starting. To make the wood filler, I find that a good ratio to start with is to use three parts yellow wood glue to one part sawdust.