Vacuum Bagging...

I've decided to build the core of my guitar body out of marine plywood.  By using plywood instead of a solid piece of maple or something, my guitar will be slightly lighter.  Also, there was half a sheet kicking around the shop so that makes my project slightly less expensive.  (If it's already purchased, it's free, right?)  Marine ply has been used for guitars before, so I am not breaking any new ground, plus the finish I have in mind for the guitar doesn't warrant a solid piece of expensive hardwood.  Lastly, marine ply is very uniform/flat with no bubbles in between the layers and is treated to be resistant to rotting due to moisture.  It is very high quality - good enough to be used on boats and docks - thus the name.

Friday, I cut a couple of rectangles out of marine ply just slightly larger than my body pattern.  I need two rectangles since this plywood is 3/4" thick and a regular guitar body is much thicker than that.  Today, I had to glue the two pieces of 3/4" plywood together to make one piece at 1-1/2" thick.  I used a technique called vacuum bagging.  Essentially, you put glue on the two pieces, put them inside of a sealed plastic bag and suck all the air out of the bag.  No air in the bag means no air between the two pieces you are gluing, which means they suck together and glue up really nicely.  We use this technique to install decking on yachts as well as to laminate veneers to desk tops and such.  It's a bit of a black art, but works amazingly well once everything is figured out.

Homemade Vacuum Pump

First, I rolled a little bit of wood glue onto both pieces of plywood.

Rolling Glue

Then, I place the two pieces together and put everything in my sealed bag and turn on the pump.

Sucking all the air out...

The towel helps circulate the air as well as protect the plastic bag from the sharp corners of the plywood.  Keep in mind that you cannot use a regular household vacuum to do this technique; there is not enough suction.  We built our vacuum pump using an old pump we bought off of some stranger and some hosing.  A regular household vacuum will pull about 1 or 2 inches of mercury (inches of mercury = inHG.  30 inHg is a perfect vacuum with NO air left).  I have seen our vacuum pump pull as high as 21 inHg.  It usually doesn't pull that high due to small leaks in the bag or where the hose goes through the bag.  In this case, I got just over 17 inHg.

17 inches of mercury (inHg)

A quick calculation tells me that 17 inHg equals about 8.35 pounds per square inch.  That means that for my two pieces of plywood, I have the equivalent of a weight of about 1600lbs sitting on top of them to glue them together.  It would be difficult to clamp it that tight, not to mention that evenly.  The greatest thing about vacuum bagging is that the pressure is completely even.  There are no loose spots.  I turned the pump off after a couple of hours and tomorrow we will see if everything glued up alright.