Having been in this same situation, my experience was that powder coating creates about three times thicker layer than normal paint. Good for durability but not so good for mounting surfaces like BB and headset. Even with a cartridge BB, it is necessary to face the bb shell to get a good seat with the fixed "cup" side. And if your cartridge bb uses lockrings on the adjustable side, you'll have to face there too. I originally mounted the BB (Shimano UN-91) without facing the fixed side and the gap was so significant (even after cranking down) that water and grime got into the bb through the gap and then the threads. When I pulled the bb a year later, I had surface rust. Ended up with much more work to clean that mess up. So good call on getting the facing done before all that nonsense happens.