Prep for painting an old, interior, coquina rock fireplace wall?
So I jumped on YouTube to refresh my memory on dealing with this. 

For a while people were just demo-ing them altogether. 

You seem to implying an intent to fill and cover the entire surface for a smooth effect. Those projects tend to use preblended colored mortar (just add water), and they trowel it into all the cracks and holes.

The newer thing I found was people chiseling down and whitewashing (with a sprayer) the stone. Because of the application it covers all the surfaces for a very uniform updated look. 

I would definitely send you to YouTube for more ideas if nothing else. 
Search "Fireplace Makeover"