Painting an unknown plastic can sometimes do more harm than good as the
solvents in the paint may cause the plastic to stress relieve itself
into crumbling powder. At the very least, test a small spot with the
paint you want to use. Let it sit for a couple week and observe
carefully.

If it were me, I would coat the enclosure with epoxy as a barrier coat
and then paint. I use porch and deck polyurethane for projects like
this and get good service out of it.

Ron