Probably atop a fully cured , solid coat of an alkyd primer , like the Rusto 2X
I've never used the stuff but it's a solvent fest in a can .
You should test that hypothesis out on scrap like you say ,
I've never used Tamiya's Polycarbonate spray, but I've asked my LHS clerk about it.
He said that this paint is designed to be more rugged on R/C car bodies so that the paint doesn't flake or chip off during collisions with obstacles and other cars.
Apparently, regular lacquers and enamels will flake off the inside of smooth clear polycarbonate R/C car bodies.
But I would try spraying some on a styrene sprue first to see if does anything bad to the plastic.
I would test that too, if I were you. For all you know, the polycarbonate paint will react poorly to the primer underneath. Don't test it anywhere you already applied the polycarbonate paint to bare plastic already, BTW, because then you don't have a proper test of what happens with only primer plus polycarbonate paint.
Also: why are you testing this on a sprue with parts in it?