No primer necessary, shellac will leave shellac is it is stripped. But
if it is painted, thats it, it is painted. If you want a color that can
look like stone or concrete and can easily be lightened to a lighter
stone color with White, get Benjamin Moore Briarwood. It is a color
that can truely easily match to look like stone or concrete. I have
used it on many jobs. Even white painted concrete- stone as in painted
white window ledges to make them look natural again. Then you could
Faux finish the fireplace with Briarwood as the base if you want
something more.