The commercial stuff can be a pain but there are different kinds. In any case, you will have to do a lot of layering and/or sanding to get a glass smooth finish. I've tried using a filler once and I seem to remember that the most important step is sealing the wood prior to filling. The wipe-on variety of sealers and fillers are what I've used. I don't care for the kind that you spread on with a knife or some other stiff applicator.