Seems a shame to paint marble but I guess if you really hate it or something, have at it. You might be surprised by an ad in Craig's List. Somebody might love to have it and you can use the money for a wood mantle and surround or something. Stone and marble are the craze right now.
Or take it down and cut it into pastry boards or insets for the kitchen if it is thick enough. Or something?
Once you paint marble you will render it more or less useless if somebody wants to restore it. Either the paint or the process or chemicals needed to strip the paint off will stain it.
If you must paint it: Wash thouroughly with TSP and rinse completely. Use an adhesion-promoting primer, such as XIM-UMA. This can be top-coated with whatever paint you choose.
[start rant] I watched a DIY remodeling show where they were fixing a 100 year old house that had beautiful vintage wood throughout, including a mantle. They painted virtually all of it white. I wanted to throw up. [end rant]
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