After some more work shaping the foam, I was ready to seal it. The main issue was that Iíd shaped it a bit too far on the sides in the middle, losing some of the compound curvature that you ca see in the top center. I needed to build that up and fill in some gaps that remained in the foam mold.
Drywall compound is the answer. It allows a build-up, the consistency is the same across multiple layers and that avoids sanding discontinuities. Also, itís cheap and I didnít have to mix it. Normally, I would not permit this anywhere on the plane, but since this is a mold, not a flight part, itís okay here.
This is after Iíd just started on the right side. Previously, Iíd verified that the drywall compound will stick to the foam. I'm just using a yellow plastic epoxy squeegee for this instead of my drywall tools. They can't handle compound curves.
And hereís the other side ready to dry.
The instructions say to apply it in relatively thin coats to avoid cracking as it dries.
For actual drywall, Iíd try very hard to smooth the surface at this stage and avoid sanding. In fact, thatís a difference between a drywall pro and a beginner. The pro doesnít need sandpaper, or at least not much. Here, with the compound curved surface, thatís difficult, and also I donít need a finished paint-ready surface since itíll be covered with plastic.