According to DIY Network, the first step in painting an indoor concrete slab is to wash the area with a de-greasing solution and allow it to completely dry prior to painting. If the slab shows signs of water leakage, it's important to determine the source of the leak and correct the issue before continuing on with the project.
After the slab has dried, sand the slab to smooth over uneven areas and remove traces of old paint. Then vacuum the area and wipe it down with a tack cloth. The next step is to prepare a concrete patch compound and fill in any rough spots, holes or cracks. Allow the concrete compound to dry thoroughly.
Once the concrete compound has dried, the next step is to apply two coats of paint primer with a paint roller. It's important to allow the primer to dry completely after each coat. After the second coat of primer has dried, apply two or more coats of epoxy paint to the slab with a paint roller or paint compressor, allowing each coat to dry between applications. Finally, apply three or more coats of masonry sealer to the entire area. Allow the slab to dry before utilizing the surface.