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This technique was adopted directly from model rocketry construction.  It is a way to make strong glued joints for various woodworking projects you may work on. 

Step 1: Materials

 - 2 pieces of wood (using wood shims here)
 - Wood glue (or Elmer's would work)
 - Spring clams or C-clams (Home Depot has them)

Step 2: First Layer of Glue

1.  Put a layer of glue on each piece of wood.
2.  Spread out evenly on each piece with your finger or brush.
3.  Let it completely dry.

Step 3: Apply 3rd Layer

1.  After the glue has completely dried, put a 3rd layer of glue on one of the dried layers.

Step 4: Clamp Pieces Together

1.  Before the glue dries, clamp the pieces together
2.  Wait until it dries before removing clamps.
3.  Fill in spaces with additional glue.

Step 5: Schematic

Quick schematic of the double-glue joint.
It should hold together strongly.
This technique works only if the first layers of glue have not fully cured. PVA is wood glue not glue glue and if the first layer has hardened off (which actually takes a few days) then the bond will be a very poor one indeed. The best bonds are where the surfaces are clean and freshly cut with a plane or cabinet scraper (Too fine a sandpapering can actually tear the fibres of the wood, block the pores with dust and char the surface). PVA works best with high clamping pressures and tight joints, and is actually the strongest glue in that circumstance. This double glueing can compensate for joins not being tight but you may be better off either doing a better prep job or grabbing the epoxy, which spans poor joints better. <br> <br>The technique attempts to mimic the old hide glue method of sizing (or priming) the surfaces with glue before glueing. Hide glue does stick very well to itself but PVA does not. The technique will not give as strong a joint as just gluing and clamping on a tight/flat fit like those shims you are demonstrating with. <br> <br>To get the best grip, ensuring the pores fill with glue by rubbing it in with your finger (or some thing like a printer's ink roller) works really well but don't bother waiting for the glue to fully dry, just add a little more to have some wetness in the join and clamp well. <br> <br>That being said the join you have made would be fine for most things but would not be as strong as it could be. <br>
One application that I have heard off (yes, in a youtube-woodworking-video) is to apply a layer of glue to wood that is very &quot;thirsty&quot;, i.e. sucks the glue up, like endgrain on some woods. This is meant to saturate the wood a little before doing the actual glue-up. Apart from that, I second what Titch said.
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