Hey all. We have a early to mid 80's plaster over gunite. We had the main drain replaced last year and I noticed some hollow sounding spots in the plaster. I don't have any chips but I do have a few vertical streaks in it. Just curious on the life expectancy of plaster. Ball park cost on a re surface and what the hollow sound was. De- laminating plaster?? We are in southern NH so we close and freeze every winter.