what do we need to do to seal the slate mantle ?

Hi there --

We just stripped three coats of white paint off a black slate mantle 
piece in our new house with a store-bought paint stripper.  Now the 
mantle has a slight white film -- just a bit of paint residue -- and (http://maplered.com)  
looks more like a pale grey blackboard than black slate.  What can we 
use to strip off the remaining paint?  We've seen so many different 
suggestions on the bulletin board, between soap, acetone, and (http://maplered.com)  
everything else.  Also, what do we need to do to seal it?

Many thanks,

Matt & Jenn