I have an old guitar that has a fairly good lacquer finish, The soundboard has 3 scrapes not very deep but the finish is mostly gone in the scrape (2-3" long). My problem, they have accumulated dirt over time in the wood grain. How do I properly clean before repairing the scrapes? I also would plan on a light layer of shellac before adding lacquer to keep from having 'stain' issues where I replace the lacquer, or is that necessary?