Number 1 problem is the discs not mating. Because they are not mating and there are no channels, vacuum pockets are forming and locking the discs together. STAY ON #240 UNTIL (1) locking ceases and (2)sharpie test shows uniform mating AND (3)sag is close to 3mm and (4) there are no pits anywhere. (yes, I'm raising my voice
)
Sharpie will grind off at 5um