Every dial is different. Sometime we read here people wanting to clean corrosion or substrate reaction, or faded paint - you can't do that. While a damped earbud, applied gently, is almost always safe, I have seen cases were it was not. So avoid any chemicals and try first to understand well what is dirt and what is not, what the dial can take and what it can't. Of course, experience helps, when in doubt post good picture here as good advice is always present.