Over the years, all the harp companies have used a variety of finishes: varnish, lacquer, Poly, Epoxy, etc. For years, one of the major companies used a finish that absorbed the color of fabric that touched it. So there would be dark spots where the knees touched and where the shoulder touched the instrument. It couldn’t be washed off with either soap and water, or any solvent. It had gone right through the finish into the wood. It’s possible that what you are seeing on your harp has absorbed into the finish. If soap cleaners of different types have no affect, then try a solvent, like alcohol or lacquer thinner on a small spot and see if that has any effect. If it doesn’t, then you might try abrading a spot with very fine sandpaper to see if it is just on the surface. If it is, then you should be able to polish it off. If it isn’t, then you might just have to live with it. You might try a cleaner called something like krud-kutter. You can get it at Home Depot. Good luck!

I discovered that a tiny bit of Ivory bar soap really cut through that grime, on a slightly damp cloth, rub gently.