Assemble then paint, almost always. I paint for tabletop standard, if I can’t reach it it can’t be seen at tabletop distances. There are some rare cases where I have to paint then assemble. I’m going to put together four 1/48 uh1 hueys where it’s easier to paint first than it is to mask off the clear canopies. But they’re models and not wargaming pieces so it’s an exception.