Q&A for How to Glue a Puzzle

Gluing one side of your puzzle will hold the pieces together, and gluing either front or back can work for this purposes. However, gluing both sides will provide the greatest stability and prevent pieces from coming loose.

A little might, but if you use PVA glue or another type of clear glue it's unlikely it will be noticeable.

One side isn't superior than the other, but gluing only one side can result in a slight curl to the pieces. This can be fixed by applying glue to both sides of the puzzle.

It sounds like you bought gloss finish. Look for one that says "matte" (dullest) or "satin" (in between shiny and matte). If your current bottle is satin, go with one that is matte. If you want to try roughing up that super shiny surface, use a medium sandpaper to lightly scuff up the surface.

A backer board works well for flipping a puzzle over onto parchment paper. You can make this by cutting a flat piece of cardboard or thin piece of plywood, anything as large as your puzzle and flat will do. Put the parchment paper over the puzzle, then put the board on the parchment. Clamp them together and flip the puzzle over.

Decoupage glue would work for your jigsaw gluing, though depending on the brand, it might not hold up to wear and tear as well as puzzle glue.

Use some mod podge. If you carefully take a sponge brush and spread it around the 3D puzzle, it should work.

No, it wont hold up. Puzzle glue is more durable to wear and tear. Elmer's will flake and fall apart.