verdigris takes several forms on these coins, the type that sits on the surface of the coin ie hoard coins and the other main sort which form spots on an otherwise sound bronze coin, the first type is easily removed and made stable the second can be removed but can in some instances leave a crater in the metal, a usefull tip is visual inspection, the first type will be a strong green and quite firm, the second and most harmfull will be a light green and powdery type, either way verdigris unless stabilized will continue to form!

as with any deposit on a coin look at the material deposited and reverse your thinking to find a solution