I need to clean up and restring a banjo for a friend who's having some problems that prevent him from doing it on his own. Seems like a straightforward job, but I wonder: are there common pitfalls to watch out for? The head -- seems to be some kind of plastic -- has coffee or something spilled on it. Is any particular cleaner recommended? Seems also to be a dust buildup around the resonator. I suspect I should avoid using any kind of solvent; can anyone say what works best for this part of the job? Thanks