White Carrera marble statues

Can anyone help me please as I am trying to clean large very old statues made of Carrera marble, they are white and a lot of them are pitted with black spots. I have tried cleaning them with steradant which is a dental teeth whitner and wrapping in plastic wrap it is taking of the surface dirt but not cleaning the more deep grained dirt.

They are outside statues, so any information would be great.

 

Thank you,

 

Maree