Well, that didn't work. While the gray paint stuck on the silver layer okay, the yellow would scratch and peel off if you looked at it wrong. So I sanded those parts back down to bare plastic:
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Warning signs that the silver paint wouldn't take layers on top started with the tracing paper residue not sticking so well. Warning sign #2 was the liquid masking agent would bead up, not conforming to the stencilled shapes on my armour pieces.
This time around, I'll try RKD's trick of a shot of DullCote over the silver layer to make everything else on top stick just a little better.
I sanded down the right chest piece because I had sprayed the dark gray first, when I probably should have done the lighter 'base metal' gray first.