Pasting (3 strip method)
From PyroGuide
The 3 strip pasting method is a way to wrap your shells in paper in order to reinforce them to get a better break. The average plastic or paper shell is molded between 1/4" and 1/2" smaller than the nominal inch-sized mortars to allow for adjustment by the pyrotechnician for a tight fit. This method of pasting wraps the outside of the shell well, and without "building up" at the poles.
Method
The size of strips and weight of paper will be influenced by the diameter of the round shell you're wrapping. See the round shell building page for more pertinent information on specific sizes and weights.
Using your cut strips, apply a thin layer of Wheat paste with a brush and allow them to get slightly tacky before trying to stick them to your shell. If you're using gummed paper tape, apply a small amount of water with a sponge brush and immediately begin working, as the glue on these sets much faster. Don't wet more strips than you can put on your shell in a small amount of time.
Starting at one pole of your shell, lay a strip down from top to bottom. Lay a second strip down next to it so that the ends slightly overlap and the middles of each strip barely touch each other. Place a third strip overtop both of those strips and centred between them. Using a wooden dowel or other hard implement, roll the air bubbles out from under the strips.
Continue next to the second strip you laid with again another strip that barely touches it at the middle. Lay a third strip again that covers both of them equally and roll out the bubbles. Continue around the shell until you reach the point at which you started. You should now have 2 layers of paper pasted on your shell. Its a good idea to mark every 2 to 4 layers on the shell so you know how many layers are on it. It is recommended that you paste no more than 4 layers (twice around with this method) at a time until you let the shell fully dry. Otherwise, the paper may buckle or wrinkle and cause your shell to no longer be symmetrical. With Gummed paper, it is more readily acceptable to paste 6 to 8 layers however because of the papers tendency to be a little dryer. Depending on the shell, it should take somewhere around 12-14 layers (7 times around with this method) to reach the desired diameter.
It should take several hours or even days for your pasting to dry fully, depending on your relative humidity. Tip: for an easier sliding shell in and out of a mortar, using your brush, apply a thin layer of the wheat paste outside of the paper when you've finished pasting it. The paste will dry hard and smooth, much like shellac on wood. You could also use a spray sealer or clear coat to achieve the same result.