Hi Guys,  I did this job, 50 grit grind and hone up to a 800 AO honing powder, then a light coat of a solvent based impregnator. Part was interior and part ext.  After just 2-3 months, everywhere that has been exposed to rain is oxidizing (due to acid rain I believe) and the owners are getting funny, saying they want me to do it over, that it was never done correctly, even though it was inspected and paid upon completion. Has anyone seen this marble used outside? I need back up that this is a natural occurrence and I did not do anything wrong. It is quite obvious the problem because all areas not under roof are perfect, but they are trying to get it done over for free.