mullions

Hey everyone, New to the forum and wondered if anyone could help? I have a customer that I am doing some work for with building custom cabinets and restoring stairs etc. They have asked me to look at the windows. Basically they have had new aluminium windows install but they keep weeping...

Hi Can anyone tell me a good way to clean stone mullions inside please? We have some black spotting and don’t want to use anything which might damage them - so thinking no chemicals and “natural” cleaning agents? Any recommendations and methods most welcome. Thanks, Ben