Funny thing to ask about at half one in the morning.
A green dishwash pad will clean them. For proper stains cellulose thiners. This is expensive and time consuming though, do they want to pay or just want you to do it anyway.
I can bring frames up like new, the problem i've got is the customers don't realise it's down to me and think they were always that way and then complain about a minor spot somewhere else.