Durabond sets faster but doesn't cure any faster then ap mudd. Normally delayed shrinking occurs within the first 30 days. IDK exactly whats going on but treat durabond as you would ap mud. Fans, 24hrs before recoat, feathering out all seams extra wide with minimal amount of sanding. When using hot muds some days your are the dog. Some days you are the tree. Personally I finish coat with usg ap mudd. It just takes primer better and sands a butt load easier.