Instead of filling and sanding the nicks, heat up the sealing iron. place a drop of water in the nick and lay the hot iron on it. You should be able to raise all of the nicks.

With fabric and dope you still need to sand the framework before you lay the fabric. Then you also need to use fine steel wool, or 300 or finer grit sandpaper between coats of dope. Dope raises the fibers of the fabric. The first coat or two you will notice a very rough texture as you rub your hand across it.

You may want to check out the Vintage/Golden age area.