I junked the first attempt at a jacket mold.
The last can of Rebound 25 in town was at Blick, in one of those over-priced starter packs. So after laying up three coats of brushable silicone, I used the included "Plasti-Paste" to make a jacket.
I like the Plasti-Paste. Easy to work with, sets up quick. The Rebound, less so. It didn't capture the details that well, and without thinning was extremely hard to "brush on." Certainly with my fragile master it was. I might do better with Oomoo and thixatropic additive.
I came very close to dropping some chunks of recycled mold into the thing as registration keys, until I realized that one was tin-cure, the other silicone. Oops!
I made a sandwich of jacket mold, mother mold, and a backing board, and did three pours -- one cup at a time -- of Smooth-Cast 300. Unfortunately more seemed to slop up on the backing board than actually went on to the rim, but otherwise it was a successful open-mold slush cast.
Sort of. The Rebound layer was thin, and the registration keys small, and it seems to have peeled away from the mother mold during the first pour. So the final cast is a little warped. Which works okay for what it is simulating, but is annoying. All in all, this turns out not to be a great shape for a jacket mold. I would have saved time and money by just box-molding it with Oomoo in the first place.
But then, half the reason for the exercise was to learn jacket molds, and I got to try out latex while I was at it.
Little of the surface detail made it on to the final cast so to make it look more like a hammered metal gong I got out my Dremel and carved some divots. That was a mistake; now it looks like a really poor attempt at a golf ball. So I'm filling in the divots with putty, and hoping a lot that when I do the final paint job it will look half-way acceptable. I have given up hope on achieving a really nice-looking gong -- at least, not one I can have finished and in the actor's hands by Sunday.
The divots are mostly filled now, and the effect isn't bad at all; they sort of extend the depth and scale of the hammered finish. Meanwhile I prepped the mold for a possible second pull, and discovered you may be able to glue down a mother mold to a too-thin silicone jacket, but not with Superglue; it caused the silicone to shrink in the places where I applied it.
It might be vaguely possible to slip a thin print coat of Oomoo between the damaged mold and the original master, but I wouldn't count on that working.