@Chris4
I have bought EPS in densities 0.88PCF (14kg/m^3 ‘soft’ ) and 1.5PCF (24kg/m^3 ‘hard’ ).
The lighter one was an approx 150mm sheet, the heavier one was a large block 0.7 x 1.2 x 3+ meters (2x4x12 feet).
The heavier foam was easier to shape and sand and more resistant to dents in the shaping bay, which sometimes can be steamed out with a clothes iron and a damp rag.
I have never done a PU blank so I cannot compare to that. Some folks use an even heavier 2PCF foam and the ‘fused’ varieties should afford less bead tearout.
I used lightweight spackle before glassing. I don’t think (it’s been a few years) I liked the one that I added extra acrylic via floor polish to the spackle.
I have added stringers, have yet to try marine ply. I now have access to a nice bandsaw with a resaw blade so I may go old-school on the next one.
Here is my last ‘build’ thread (2015, has it been that long?) with glassing notes on the last page:
I would lean towards surf-specific epoxy to get the viscosity and brighteners and reasonable flip times. My last Ebay batch of epoxy had a three day flip time (hard enough to sand) where surf blends are measured in hours not days. Better materials do make a difference, including glass cloth and sandpaper.
Don’t consider my advice to be the end-all, there are way more experienced people here that have taught me most of what I know.