Join Date: Jan 2019
Location: Central North Carolina
If it will be painted, I usually use Bondo Automotive Body Filler. If it will be stained, I frequently do the same, but then use an artist brush and darker stain of similar color to kind of hand paint grain lines, etc across the smooth sanded Bondo. I have tried other things, but never had good luck with them For me, the Bondo works best.
Another way is to drill the knot hole to a known round shape and install a plug that is cut from the a scrap of the same wood. The special plug cutters that are available will allow you to cut a tapered plug with grain lines running across the plug. Use glue and drive the plug into the drilled hole, keeping the grain lines of the plug oriented to match the project grain lines. Drive it in until it is tight and then let the glue dry. Then saw, chisel, sand the excess wood plug until it is flat and smooth with the surface of the project board. You may need to do the decorative art brush technique to this plug too, but it won't need much if you are careful to match the grain lines of the plug to that of the project board.