Lots of conflicting advice. Some of it good, some of it less so.
We have thousands of poly boxes and we paint ALL the outer surfaces of the hive with decent grade GLOSS oil based paint with a little thinners (to the recommended level for a sprayer) added. It bonds very well to the surface of the poly and seems as if it will last a lifetime, as will the boxes. Our first wave of painted ones are now 16 years in service and most look very fresh.
Where masonry paint comes into its own is the internal surfaces of the feeders. In an external situation, water based masonry paint on poly does eventually flake a bit, a problem that does not happen using oil based paints.
But then, if you buy the recommended box paint from some poly makers they sell a water based paint, others oil based.