Last weekend I made up 12 tread and risers for a stairway using ash. Last night I was putting on the 2<SUP>nd</SUP> coat of polyurethane varnish (in the shop) and after I had finished the varnishing I notice that all the risers and tread started cupping. I used sanding sealer on the back sides of some of the treads and none on the risers and all are cupping. I cannot stack them because of the fresh finish. The past 2 days have been very humid. If anyone can give me any advice on how to flatten them I would greatly appreciate it. Also I need to apply one more coat of varnish on the top side.
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Garry