Marine Fir plywood edge adhesion

Yes ondarvr. You hit the nail on the head. The temperature changes here. It is brutal. R-value causes temperature difference but needed here more than most places. Thanks

 

I understand your reasons for stacking, although I would do it differently. I would only stack if the endgrain optic is specifically desired.

Baltic birch quality is very manufacturer dependent, there is "marine" quality and there is garbage, all you can do is try it out and find a consistent supplier. I for example can find good 50mm baltic birch ply without much trouble (good as in it exceeds the B/BB face grading and has minimal/no voids), so if I had to build your door I would just glue two sheets together (no clamps or jigs, either weigh it down or use screws in a grid pattern) and be done with it. Any small raised carvings either glue them on already carved in another piece of ply, anything below just gets grinded into it (by CNC if available). If a huge complicated carved design is necessary I would glue three 50mm sheets together and just stick it into the CNC.

For sealing the endgrain (wich would also be required with my approach if carved) there are two ways: use a thin and slow epoxy to saturate the surface, then follow up with thickened epoxy as a filler, or use a water thin cyanoacrilate glue, followed with a filler of your choice. CA is usually more expensive and only used for small surfaces.
Fiberglassing a carved surface would be a nightmare even vacuum assisted, so if an irregular surface is mandatory with softwood ply, I would fiberglass the base sheet and glue on the carvings made in something else like epoxy foam.

Expanding foam is PU/PIR the only difference between pour in and rigid is that the rigid has consistent density and quality because it is expanded in a factory under controlled conditions. Glueing is by thickened epoxy or polyester (depending on substrate). The only problem is that high density rigid PU is usually a specialty item, you need a good supplier and have to pay shipping, wich might prove to expensive.

 

Thank you Rumars. Got me thinking for sure. Here are a couple of questions that would really help me out as it seems I’m headed toward a traditional type build.
2” Baltic Birch. I would need to search that. Maybe hard to find on this side of the pond. If not Baltic Birch which ply is best?
Your method leaves a solid door no insulation. I know there is not much insulting in a door anyway because of its thickness. But you think this is ok for exterior super cold conditions? Plys glued together like this offer good stability as opposed to stiffeners and frame? Thanks Rumars, very good advise for sure.