What is the best way to attatch plaster cast rock to foam.
Acrylic ceramic tile mastic cement. Consistency of peanut butter, easy to apply, holds firm. Clean up hands and tools with water; but once it sets, it's set.
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I always have a tube of LePage's PL300 handy, and that is what I have used over four layouts to date. It is tacky and pliable, almost like taffy, but it will harden oveer time. Light plaster castings won't sag much, but you would be wise to place something below the casting to help keep it put.
I just used foam compatable, cheap laytex caulk. Worked fine, lasted over 10 years, seemed to still be holding when I sold the layout
Good luck,
Richard
Latex caulk, never been a proublem.