Guess that’s not possible yet… When I join multiple surfaces (or meshes), it would be handy if the resulting object had the same materials, but per-face. In other words, per-object materials are converted to per-face ones. Optional.
There is a fundamental reason for this not being possible at the moment, but I’ll need @nathanletwory to chime in as to the actual technical details.
Probably same as RH-73290
Keep materials and mapping when joining objects. Adding this thread to the item.