I painted the cedar siding last year. Now there are many areas that the paint is blistering. It looks like there is water in the blisters. The siding had been painted many times before. I scrapped amd sanded the old paint. The guy at the paint store told me is is trapped water behind the paint. How do I get the water out??? Please help! I am getting mixed to no answers from the contractors here. I live in Pittsburgh Pa.