Another tip for foam, after lots of experience, is to always have a sharp blade and use a straight edge. Starting with a perfect cut will save tons of fixing time later. The first time you feel drag, change the blade. And you will go through a lot of them.
For fixing and filling seams during the coating process, I use flexible caulk. ElastoPatch, I think is one brand from DAP that’s available at most hardware stores. It’s not sandable, but easy to smooth with your finger and some warm water.