How can I put the white back in the line?

Dear Sirs:
 I have a slate topped coffee table that has a design of a large boat
engraved in the top. When I purchased it the lines were white  and (http://maplered.com) over
the years and (http://maplered.com) with various cleaning ladies who liked to spray lots of
aerosol cleaners about the white has gradually disappeared. How can I
put the white back in the lines to better show up the design.
Thank you ;