You can buy lots of fillers for disguising the holes that remain after countersinking finishing nails, but these often leave telltale results.
woodworking technique shop tip

From a piece of scrap that matches your work stock, use leather hole punches to cut tiny plugs to match the size of the nail holes. Set the nails slightly below the surface, and dab a little glue into the indentation with a wood sliver or a paper clip. Then, press the plug into place, making sure the grain of the plug matches the workpiece. It takes a sharp eye to spot these plugs.
—Bill Blain, Edmonton, Alberta