What is a good glue or adhesive to join Hydrocal castings & soft metal castings? What sticks to one doesn’t want to stick to the other. Thanks
Walthers Goo sticks to everything 
White glue for hydrocal and plaster castings, epoxy, medium CA (superglue) and Walther’s Goo (contact cement) for metal to metal,
To attach metal to plaster (hydrocal), I would seal the plaster first, then use any of the ones mentioned above except white glue. If you don’t seal it then I would probably go with the Goo.
BTW you can use Liquid Nails or any other contact cement (hardware store) if you can’t get the Walther’s Goo
Silicone adhesive (like bathtub caulk) will stick to most anything, too. However, you can NOT paint or stain it, so don’t get any on the surface. It also smells bad (ammonia) when it’s curing. I use it for hydrocal-to-styrene joints.