If I want to fill a square corner on a mould tool using Uni-Mould, should I use a Coupling coat / CSM "paste" at the corner?  And if so do I add the paste prior to the main coupling coat application or after?  Or is there a better way?

When using polyester resins and csm (unimould system) the best way to fill the corners is a narrow brush, or ideally a corner roller tool. Csm gets down in every corner if you are careful.