From a repairmans point of view, here is what I would do since it is new wood. Fill the area with real wood, even cut the area bigger and fill with the same kind of wood. After sanding smooth, it may look fine and nothing more needs to be done, if not, seal the area with a little shellac. After dry, use quality powdered colors that are made to be mixed with lacquer and with a very fine brush, paint the witness line out and try to match the exact color of the wood, adding grain lines if neccessary. Once the whole thing is dry, add a little color to your first coat of lacquer and spray the whole uke, back and sides with this mixture. Proceed to finish then as you normally would, sprayed lacquer is best for this kind of reapir but shellac will also work fine. That's what I would do.
