Fiberglass Armor Construction: Plaster Mold
Making the Plaster of Paris Support Mold
Once the rubber has cured you can start building a support mold for it. Is the piece is really small a support mold is not necessary, but as a piece gets larger this is a great way to give support without having to use a lot more rubber. Plaster of paris is much cheaper after all.
For this project I used FGR-95 which is almost identical to plaster of paris. In fact, when in a bind I have just picked up and used plain old Plaster of Paris from the hardware store. The main difference I can tell between the two is that the FGR-95 seems to dry faster and have a bit of a harder feel to it, but this is a purely subjective impression. My preference is to use it because I like the results and the cost is about the same, but in this article I will just use the generic Plaster of Paris name.
This section is really easy. You are going to put about 3 layers of the plaster on the piece. As with most of this work I suggest wearing latex gloves when applying the stuff.
Mix up the first batch of plaster following the package instructions. Usually these things are mixed by weight, so you will want to have a scale handy. Volume mixing does not work as the powdered plaster can compact and so a specific volume can be any weight. Once the plaster is mixed take a handful of it and rub it on the rubber mold. You want to build up a nice layer over the whole piece. How much you can put on before it starts to run depends on the thickness of the plaster and the angle. For the first layer it is most important to have everything covered, thickness will be built up in subsequent layers. Once done rinse your hands off in a bucket of water to remove any plaster on your hands or gloves. You don’t want to do this over a sink as a buildup of the plaster in the drain could stop up the sink.
For the next layer do the same as the first layer. Focus on building up the thickness.
For the third layer you will want some course water-soluble fiberglass or burlap to use as reinforcement. Either of these fibers will give the piece much more strength. If the mold cracks the fibers will help hold it together, without them the pieces would just separate. The fiber reinforcement saved me from having to make a whole new plaster mold when one of the plaster pieces cracked.
I used a water-soluble fiberglass that I got from Douglass and Sturgess. I had never heard of it before and do not know where else to get it. It did work very well and made the pieces really strong. The only drawback to the stuff is that once it dries it becomes stiff and there are usually little pieces of fiberglass sticking out of the plaster like needles. They can be covered with another layer of plaster, but I did not always do that and I probably should have since having a bunch of fiberglass needles in my hands was not fun. So if you use the fiberglass be sure to wear at least one or two pair of latex gloves when working with it or handling the molds afterwards. If you go for the burlap option you wont have to worry about this, plus burlaps is a bit easier to find, but the resulting piece will not be as strong. And may require more plaster to fully coat and reinforce the burlap.
With whichever fiber you are using you will want to cut it up into strips or squares that you can put on the mold. The trick is to not make them either too small or too large. Each piece should be able to cover a fair amount (around 10-20% of the surface area) of the mold without having to have too many creases in it. If there are a bunch of creases to make the piece fit the shape you will want to make the pieces smaller. Cut out all the pieces before you mix the plaster. When cutting them out be sure to plan for each piece overlapping the others, as that is how the fiber gets a lot of its strength. I suggest also cutting some extra pieces so you can have them handy if you need them.
For both types of fiber reinforcement the process is the same. First lay down a thin layer of the plaster, then take a piece of fiber, dump it in water and then wring it out and press it into the plaster. Be sure to overlap each piece with the other ones until the whole mold is covered and there are no gaps. Now take the rest of the plaster and rub it into the fiber and then lay some more on top so that the fiber is completely covered. It is important to rub the plaster into the fiber to make sure there is a strong connection between them. Covering the fiber after makes the piece sturdier. It is okay if some fibers stick out, but generally you should try to cover them all.
Give the support mold a good 24 hours to dry before demolding.