Thanks for the replies. I wonder if there are differences in the manufacturing process in different countries for making MDF? The MDF I use is wood pulp, mixed with a resin material, and pressed into flat sheets, with a smooth finish on one, or both, sides; some MDF sheets have a rough, textured finish on one side, which I try not to use.
I've found that the best glue, thus far in my experiences, has been Low temperature Hot Glue. I use 50mm square MDF bases for Low Density Polyethylene (LDPE) plastic Army Men figures: bonding the LDPE figures to their MDF bases with Hot Glue, creates a really strong bond, both to the MDF, and to the LDPE plastic -- very little will bond, strongly, with LDPE plastic...
My approach to making my buildings stand upright, while still being able to take them apart for storage, is to Hot Glue sections of alternating plastic straws on the joining walls' edges: line up the straws between two wall sections to be joined together, then slide a wooden dowel inside the aligned straw sections, to hold the two walls in place; repeat on all four corners; when disassembly is desired, remove the dowels from the glued-in-place straws, and the walls will no longer be held together. These are 54mm scaled buildings, of 2-4 stories tall, so storage is a challenge.
I really want to move forward with these buildings... I hope to devote some time to working on them this week. I really need to finish one as a prototype and proof of concept, before I move too far along with the rest of them. After proof of concept, I can finish the remaining buildings/wall sections, in an assembly line fashion, to speed them up as much as possible. I've printed off a bunch of signs and such, on label paper: peel and apply to the MDF sign cut-out's, to minimize painting... Cheers!