building a simple projection dome?

Recommend using a product that is endothermic or heat neutral if possible. Exothermic products change the shape of the balloon and will crack before becoming sufficiently strong to be self supporting.

Fifteen year ago we used this process to construct a concrete dome 120 feet in diameter. The process was more a fun project than a viable process. Later problems in operation created problems.

I did the same thing 20 yrs ago. I bought a weather balloon to use for the shape. I wanted a true half sphere so I constructed a plywood ring to inflate the balloon in. This guaranteed that the base of the dome was a true circle. I had to deflate a couple of time to reposition the balloon so it inflated to a height 1/2 of the diameter. Once you have the balloon inflated, spray the surface with PVA (poly vinyl alcohol) to protect the balloon surface. I used a polyester resin on mine. A balloon inflated is a very unstable surface. You will want to do your first lay-up with a 3/4 oz fiberglass mat and allow this thin layer to cure before building any thickness. This way you have a structure that will support the weight of additional layers of fiberglass without deflecting. To get the best translucency, try to use a polyester resin that is used in the cast marble industry. These are clearer than standard polyester resins used for regular fiberglass production. After you build the plywood ring, wrap the cut edge and cover the top surface with 2" masking tape so you don't stick the resin to the plywood or pop the balloon with a splinter.