I'm always welding verts during the modeling process. I've must have welded a half million verts alone when making the Resurrection, and before she was mapped, all the parts were sealed, no unwelded verts. But during the conversion process, smoothing errors popped up. Not dark spots mind you. Just the smoothing would come undone and the areas of the model wouldn't take any new smoothing. I still have that problem, and I gave up trying to fix it. When I need a ship converted to game format, I'll send it to someone with more experience in such things.