fathoming out a silicone mould cut procedure.
black base intersects fully with cyan uppers, it should do, as it was sliced from it using a planar surface !
come to join it back on after deleting the outer area (which is the silicone I pull away) and it says select surface, I select the cyan and a quick flicker and nothing happens.
clone it, place onto blue layer, turn off original layers, explode it, select the 8 internal surfaces it abutts to, join those then select it (black) and that which I just joined up and go join and bingo, then select rest of unit and join and bingo. success.
why such hassle, why didnt it join, why the ask for select a surface when I had all surfaces selected ?