I have a hollow, concrete block fireplace which I want to paint to look like french limestone. To do so I must fill in some, not all, of the grooves which are 3/8" wide and deep. I want the finished surface to be smooth but still look like stone. Questions I have are: 1. What is the best material to use to fill in the grooves? How do I prevent shrinkage? 2. Do the brick have to be sealed? With what? 3. What is best type of plaster to use for creating the finished semi-smooth surface? 4. Should I anticipate any problems using oil-based paint on the surface? Thanks for your thoughts.