mineral spirits will safely clean a shellac finish. Is it really shellac? If a swab of alcohol makes it tacky, it is. How rare, and, I think, pretty neat. The Formby product that Henry mentioned is still available, and another is Howard's Restor-a-finish (a great product), BUT, these are better for varnish type finishes. Shellac is kinda out there by itself in the world of finishes. What you are talking about doing is to reamalgamate the finish. Straight alcohol will dissolve and respread the shellac,but it may thin it too much and end up more like stripping it. You might need to add more material. If so, the best thing to use is more shellac, maybe a 1# cut (concentration) or so. It will dissolve the old and blend in to add body . All depends on the condition. Brushing or padding shellac is pretty tricky , but once you get the hang of it, its one of the neatest finishes around.