Pour a proper amount of glue into the bowl, depending on how much fake barbed wire you're making. Pour the exact same amount of water into the bowl. Mix the water and glue until it is smooth and looks like one substance. The water will ensure that the mixture and fake barbed wire are easier to work with.
Add the grey paint to the mixture. The diluted glue to paint ratio should be around 4 to 1. Use about 1/4 as much paint as you have diluted glue.
Cut off a piece of string that is roughly 2 times longer than the amount of barbed wire you want. The string should have a twist in it by construction. Tie one end of the string to a stationary object. Take the other end of the string and begin twisting it in the same direction as its original twist. If you twist it in the opposite direction, the string will unravel.
Continue twisting the string until the string can twist around itself. At this point, fold the string in half. When you fold the string in half, it should twist around itself. Once the string has twisted around itself, you can evenly space out the twists along the length of the entire string. You now have the first piece of your barbed wire.
Dip the string into your glue mixture. Make sure the entire string is covered and that none of the original color of the string (or the string itself) is showing through the glue. After covering the entire string, either hang it up or put it on a surface that it will not ruin to dry. Ensure that no parts of the string are touching or the pieces might get stuck together.
Take the remaining piece of string that is left and cut it into 10 cm sections. Dip each section into the glue mixture and ensure that it is completely coated. Wrap the sections around the string of barbed wire you made earlier. You can space out the barbs to whatever length you desire. The barbs will try to unwind from around the wire, so make sure to lay the wire on a flat surface to hold the barbs in place until dried. You may have to do a few barbs at a time, to ensure they don't unravel while attempting to do the next barb.
Allow all pieces of the barbed wire to dry. Once the barbs have completely dried, cut their ends at an angle. This will give the string barbs a sharper, more menacing look.