you could try this
Lithofin Stain-away
Special Remover
Disolves Hardened Wax Layers, Sealants, Stubborn Oil Stains And Grease Residues, Coats Of Paint And Lacquer, etc.
Coverage: ca.4mē/500ml Lithofin Stain-Away May Be Used With All Varieties Of Stone, Wood And Metal. It Is Particularly Well Suited For Removal Of Stains Even From Porous Materials Such As Honed Limestone, Marble etc.
but before you use it
make sure you test it out on a small piece first...
some one else many have a different idea, so stay tuned........