Grain filler is usually the consistency of thinned paste, much like rubbing compound. Wood filler is generally more like dough, thick and workable but not thin enough to really spread into the grain. It is used to fill holes generally. BUT you can thin Elmers wood filler with water, and make it work like a grain filler, and quite well, but you must work fast using this method, as it will dry and re set quite quickly. Only use a small amount of water with a small amount of filler and mix well, then spread on your chosen area working it into the grain with a small paddle or squeegy. Always work across the grain to get best fill, and do a final squeegy with the grain when finished filling area to smooth.
As for finish on Swamp Ash(Black Ash)if filled with a natural or clear filler, finish sand to 400grit, take a damp rag(NOT WET...just damp) and wipe the complete body down to raise the grain/fuzz it up, let dry for 8 hours, sand back with 400 until smooth, clean away all dust, lay down 3 coats of sealer, let dry overnight(minimum 12 hours), sand smooth with 320 and 400, you are ready to begin applying your color. Complete finish regimine from here.