Started sanding the first side of the firewall. The PVC foam glued inside the cutouts has to be perfectly flush with the 18mm Okoume plywood. I sanded for four hours and still not done with the first side.

I think the sandpaper is of low quality.

Sealing First Side With Micro-Slurry

Besides sanding, I also used the time to seal the first side (which is already flush) with dry micro . I did not take pictures unfortunately but the process is simply. Apply the micro with a spatula and spread well over the whole PVC foam surface.

The micro seals the PVC foam surface and prevents it from taking up too much resin when bonding the plywood skin.