This tortoise shell was built for our daughter's youth theater's production of Sleeping Beauty. It is made from cardboard and covered in fiberglass.
Step 1: Getting started
I found an image on the internet from which to model the shell. I took measurements of the actress from the top of her next down to about four inches above the back of her knee. It was about 34 inches. I figured a good proportion of width to height would be about 2/3 so I measured a 34" x 22" rectangle on a piece of cardboard. The shape was drawn freehand. I simply rounded the front corners and then tapered the back to make it wider in the front by about an inch. To make the rounded shape, we determined that about six inches would be enough to give dimension without being too cumbersome to wear. I cut a 6" wide piece of cardboard and curved the front and back to make the shape of the shell. The front scooped down and then out to make the collar area.
Step 2: Building up the shell
I used the same technique of building the shell as you would with a model airplane wing with ribs and covering. Each rib was simply shaped and glued in place. There are no hard and fast rules. The collar area had a thin piece added in an arch to keep track of where the neck opening was going to be placed.
Step 3: Adding the shell covering
Each section of the shell was made by gluing a sheet of cardboard over a single cell of the ribbed structure. The cardboard's corrugation was broken down to get a smooth bend along the ribs. Do this by grasping two ends of the cardboard and running it along the edge of a desk. This will break down the inner corrugation and 'soften' the cardboard. Simply work each section. I usually made the sheet a bit larger than it needed to be and once the hot glue had cooled, I used my razor knife to trim the covering. You can also see the smaller triangle sections added around the collar.
Step 4: Adding 'scutes'
The polygon shapes on a turtle's shell are called scutes. I made these by cutting and fitting the shapes to fit.
Step 5: Hollowing out the shell
In order for the shell to fit closer to the actress' body, I trimmed and peeled away the base and cut the ribs down. Up to this part, I had about three hours worth of work.
Step 6: Fiberglass
** IMPORTANT ** It is extremely important to do this outdoors where it is well ventilated as the fumes are very harmful. I wore a mask to keep from inhaling any fiberglass shards. I also wore disposable rubber gloves and a good pair of safety goggles.
The fiberglass work took about eight hours over four days. You must work in smaller sections and take your time, especially around the edges where the fiberglass bends around. It is tedious and time consuming.
You will need a box of one inch disposable brushes. You will use one for each batch of fiberglass resin. I made a total of about ten batches of resin. I tried to use foam brushes but the resin simply ate into the foam and destroyed them.
Start by cutting a short section of fiberglass mat, about four inches wide and ten inches long. Work the fiberglass from the inside and then bend it over and work it down. Take your time to push and stretch the fiberglass mat around the bends. While it cures, you will constantly need to keep pushing the mat down as it will tend to try to lift. Use quite a bit of resin so the mat is completely saturated and just keep pushing the brush into the nooks and crannies and bends. At first the fiberglass mat will seem to not want to conform, but with patience and perseverance it will work just fine. Don't get frustrated. The resin will set up in about five minutes. That is why I recommend only working with small sections when doing the edges.
Once the edges are done and cured, you can use fiberglass cloth to cover larger sections of the shell. It was able to do the top of the shell in four sections. Simply saturate the cloth and work it around the edges of the scutes.
After it was totally cured, overnight, I used 40 grit sandpaper on a block to knock down any of the sharp fibers and rough edges.
Step 7: Spackling
I usually use bondo to smooth the surface and fill the texture of the fiberglass cloth and mat, but I didn't have the time so I used vinyl spackling. I slathered it on and then used my hands (with gloves on) to smooth it all over. I didn't keep it very thick so it would resist chipping. Once it was totally dry, I hit it with some sandpaper and then used some leftover spray paint to give it a harder finish.
Step 8: Backpack
Since the shell was going to be worn like a backpack, I had the actress bring in an old school backpack that I could cut apart. I just cut the pack off and kept the back and straps, including the waist strap. This was held up for fit and then I hot glued it on.
Step 9: Painting
I used house paint from the scenery shop. The director wanted the shell to brown with gold/yellow details. I gave it a base coat and then added accents to the scutes. A sponge was used to give it the blotched look. I painted the inside of the scutes with the gold/yellow color and added some lighter swirls to give it some dimension. The grooves were painted black and then the sponge was used again to tone down the black and blend it in.
In total, it took about 15 hours to build. Build cost was about $40. This covered the cost of the fiberglass resin and cloth and spackle. I used hot glue sticks, gloves, and disposable brushes that I had from another project.