One of the most overlooked and easiest to accomplish with not being a professional painter, light,light, light coats, it is very easy to spray one more swipe because one little spot looked light. But it will always be better to use several light coats.
I don't have a lot of experience painting styrene, but have paint much fiberglass boats and other projects. I've switched to waterborne paints spraying with gun and airbrush. Using 2k clear coats. This is a pic of auto air waterborne paint with a 2k clear. It hasn't been polished yet but still pretty shiney.
And one of my trucks I've painted with same paint but not sanded and not cleared, just soda blasted the styrene, primed (sanded a little on the primer) then several coats.
The best part is I can paint in garage and be fine on inhaling. But the clear coat definitely need mask, filters, ventilation ect. And clean up is taking tools into kitchen and washing it the white sink. (But again, clear is not water base so none of that easy stuff applies to it)