Corundum is notorious for pitting with 1200-3K plated laps, and the fact that your laps are worn doesn’t help. I’ve always had good results on BATT, roughing with 325 grit if needed, else 600, fine cut with 1200, prepolish with 3K, then polish with 50K. More recently I’ve used 100K Blakstik on Diamatrix for polish.
I recently got a set (600-1200-3K) of sintered laps, but haven’t yet cut any corundum with them.