I have an INTERIOR coquina rock fireplace wall that I hate, hate, hate. Background information: Coquina rock is a very porous rock made of lots of little seashells compressed together and is found in Florida. It is extremely porous (think lava rock). I've decided to try to paint it and expect to use a several coats of bonding primer before I paint. The "mortar" or concrete, or whatever the buffoons (who thought this would be really cool back in the eighties) used to construct this is often non-existant on the face of the rock wall. It appears as if they wanted it to look like the rocks were free stacked as no mortar is present over much of the wall and this leaves really deep holes between the rocks in most places. None of the rocks are loose. It is just a really ugly brown rock wall and the surface is alarmingly uneven with some rocks jutting out maybe 4" farther than others. So now my question/s: What to do about the cracks? Most cracks and holes are too large to caulk; some are too small. Would you attempt to mortar all of the cracks before painting? What kind of mortar? Since all of the cracks are irregular, normal tuck and point is just silly, isn't it? 
Prep for painting an old, interior, coquina rock fireplace wall
cracksfireplacepaintpreparationrock
Related Topic
- How to protect the walls next to a 24 foot tall red brick fireplace when painting it
- How to prepare this old exterior wall for painting
- Retaining wall back fill/base rock and pool base rock
- Preparing friable plaster for painting
- Walls – For painting these walls, should I just paint over it the new color
- Tools for painting a wall
- What do these cracks in the drywall indicate
Best Answer
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"