Posted by busymom0 3 days ago
As a warning I'd recommend to stay away from most commercial puzzle glues. Some of them are water based - and this doesn't go well with cardboard. Anything bigger than 1000 pieces will start to warp a lot.
1 - https://en.clementoni.com/collections/adult-puzzle/products/...
Apparently Mod Podge is a brand of craft products and adhesives: https://mod-podge.com/
Or sheet of hardboard and a stanley knife, which is probably the most sensible option in context.
I found that a coating of quick-drying, low-viscosity liquid glue on the front side works pretty well, because the glue easily flows into the gaps between the pieces, and then into the space between the puzzle and the board. Once the glue dries off, all the pieces and the backing board become a single chunk of cellulose-infused plastic. This is probably much better than either the thick mod podge or the adhesive sheet that OP tried to use at first.
What people may not realize also is that at this "scale", it can happen that a piece perfectly fits in more than one location. And I don't mean a couple of cm apart, complete opposite sides of the puzzle. Fun times figuring this out.
This is the one https://m.media-amazon.com/images/I/81C6LLOhsgL._AC_SL1500_....
The solving part is fun but the later handling can use some work to be optimized.