How to build a one-piece
folding picnic table out of 2x4 lumber

Step 12. Keeping the tabletop boards even

This is a method I have found very effective (in this scenario) for keeping the tabletop boards and the seat boards even in the middle. Simply nail 6"(150mm) galvanized nails through the edge of one board into another.
Pre drill the hole first the full length of the nail and the same diameter as the nail.
Make sure the nails in the tabletop are staggered, to ensure one nail does not run into another. See the pictures below.