I've started painting the interior bare concrete block walls of a large garage, with 600 SF of wall space. The blocks are heavy and hard, made around 1950. Each block has 50-100 pits on its surface, varying from pinhole to the size of a large nail hole. This appears to be the way the blocks were made.
I painted a test patch. Two coats of quality block filler, applied with a 1.25 inch nap roller, covered only some of the holes. A coat of paint put on with the same roller covered a few more, but the pits, even when paint has gotten into them, cast shadows that look like black spots across the surface of each block. I think many are just too wide and too deep for the block filler to work.
I tried spraying with an HVLP which didn't really help.
Applying paint with a thick brush does the best job at covering the pits, but I'm not anxious to paint 600 SF, both primer and paint, with a brush.