Separating the Sheets that Make Corrugated Cardboard
Here is a side view of a sheet of corrugated cardboard:
You can see that it is composed of three distinct layers. There is a bottom flat layer, a center crinkled layer, and a top flat layer. That middle layer is called the fluted layer.
The beautiful thing about cardboard is that these three sheets are glued together with starch, typically a corn based starch. And this means it is easily dissolved.
So, you can apply a liberal amount of water to the top layer, soaking it well, and then you can peel off that top layer.
This picture shows the top layer has been gently peeled off.
You can then flip it over and repeat the process to peel off the bottom layer.
Sealing Cardboard
You can make cardboard much stronger and give it a certain amount of water resistance by sealing it with one of various products.
An easy an inexpensive way to seal cardboard is to use mod-podge. It is non toxic and water soluble but will give the cardboard strength and durability. There are many types of mod podge. For an overview consider two types: Gloss and Matte. The gloss will dry with a shiny look and matte will dry with a dull look.
Mod Podge dries clear in approximately 15-20 minutes and can be sanded to a smooth finish. Apply with brush or sponge applicator. Close container after each use. Clean up with water while wet. Once dry, Mod Podge cannot be removed. Use as a puzzle saver, a sealer glue, a quick finish, a textured finish,or a build-up finish." Non-toxic.
Shellac is a terrific way to strengthen and seal cardboard with. It is all natural and when it is dried it is completely non-toxic. This is because it is the excretion of the Lac bug. I use it often and when using it you should do multiple coats allowing it to dry between coats. Shellac Love on Amazon right here
Gluing two sheets together
When adhering two sheets together. And this is particularly critical when using a spray adhesive. It is very easy to not get them placed exactly right. Then it is difficult to re-align and shift them because of the adhesive.
You can do it effectively by applying the adhesive then hinging them together. It will give you a perfect placement without the need for realignment.
Using a Liquid Glue or Adhesive
A technique for getting an exceptionally good bond between sheets of cardboard is to rub them against each other and swirl them around just. What I mean is that you apply a liberal amount of wet glue to a cardboard surface. Place the second piece flat onto it then move that second piece around a bit (swirling it) before stopping in the final desired position. This swirling motion will get a uniform layer of glue between the two pieces causing a strong bond. In this quick video I show you what to do:
Curving Cardboard
It is quite easy to do. You just need a little patience, or a hair dryer. With this technique you can easily get some curve out of a piece of cardboard. It's perfect for some projects like making a cardboard shield that is curved.
I showed you another technique called ribbing where you can get a much larger curve out of cardboard. But with that technique you need to build some kind of a support structure to hold the cardboard in the curved shape you desire.
With the technique I show you here you get a smooth curve that holds itself in it's curved shape without any support.
The cardboard will only curve in one direction, and this is determined by the direction of the corrugations. In this picture we are going to curve this cardboard and I have drawn lines on it to show you the direction of the corrugations. This cardboard will only curve the top and bottom in. It will not curve from left to right.
TO get it to curve you apply a liberal and even coat of water, with a brush, to the top surface. You want to wet that top layer of paper. You do not want so much water that it soaks through to the bottom layer of paper.
What will happen is that when that top layer of paper dries it will shrink. And in shrinking it will curve the whole sheet of cardboard. If the bottom layer of paper were wet the whole thing would shrink evenly and the cardboard wouldn't curve.
Let it dry for a while or use a hair dryer/heat gun to speed up the process.
Bends and Folds
You can bend and fold cardboard, of course, by first cutting a line partially through it. The goal is to cut through the top layer of paper and the corrugations but not cut through the bottom layer of paper.
The thing to be cognizant of is that once you have done this you can fold the cardboard either way.
This picture shows the two layers have been cut and the cardboard has been folded in a way you would expect.
But you can get a very nice look by creasing that cut with a ball point pen then folding it the other way. This picture shows creasing with a ball point pen.
Now folding it the other way looks terrific on the outside of the fold.
Just consider one thing! The direction of the cut and the corrugations. It turns out quite different if you cut in the same direction as the corrugations. You can still fold it either way. It ends up looking different.
Bonding pieces together with the top layer.
You can wet the top layer of cardboard and peel off the paper. This is called kraft paper. You can also buy it in large rolls.
Once that kraft paper is dry you can use it to assemble cardboard pieces together like this. Cut a strip of the kraft paper and use it to stitch together two pieces of cardboard like this.
This also works for corners and bends.
Another glue/assembly technique is to cut away part of one piece (top layer and corrugations) leaving the bottom layer. Then you use this bottom layer as a flap to glue to the other piece of cardboard.
An easy an inexpensive way to seal cardboard is to use mod-podge. It is non toxic and water soluble but will give the cardboard strength and durability. There are many types of mod podge. For an overview consider two types: Gloss and Matte. The gloss will dry with a shiny look and matte will dry with a dull look. 
I showed you another technique called ribbing where you can get a much larger curve out of cardboard. But with that technique you need to build some kind of a support structure to hold the cardboard in the curved shape you desire. 
