I've had no problems using drywall over hydrocal. In fact
my SOP is to lay down the first layer using hydrocal, then
finish up with a layer or two of drywall compound. I will
add a small amout of water to the compound to thin it.
Then apply it with a brush. Rock castings get stuck into
the first layer and the second is used to blend.
Just my $0.02 worth. Hope it helps
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