We been having issues with paint sagging in our new house and have read everything we can to try to fix the issue. We've prepared the walls by cleaning with TSP (rinsing the walls of the TSP as well) and light sanding. We are currently painting primer on the wall (doing 1 coat of primer due to the dark color of the existing paint) at 52% humidity and at 72 degrees. Just to make sure the humidity is perfect, we have had a dehumidifier working in the room for the past few days keeping the humidity at 50% during the entire time. Some but not a lot of water has come out of the air. We have let the previous coat dry for a few days now.
Walls were previously painted about 5 years ago with Dutch Boy latex paint. We are painting with a Kilz 2 latex primer and we are using good Purdy brushes and a 3/8" ply professional quality roller. We are painting thin, even coats and sanding between coats. Would there be anything else causing paint sagging that we have not already tried to fix? We had some similar issues in other areas of the house using Sherwin Williams Pro Classic on the trim, but this issue is happening both on the trim and the walls. We had professionals come in to finish some painting in these areas and they had similar issues, but obviously made it look better than we could as diyers.
We have both painted successfully in the past doing less to the walls than we have done for this project so don't know what else to do.