Managed to file them down to size using needle files (that's all i have

Still didn't get the hang of it tho after picking the same practice lock for 2 weeks, sometime it takes me 5 mins sometimes by luck it's less than 30 seconds.
The only pick I've filed down is the hook pick and I still have 6 picks more to file down to the right size. I don't find any trouble filing it down to size but how do you thin down a pick effectively? I find it really frustrating to thin it down with needle files. Would a sandpaper works better than a file on thinning them down?