What grit sandpaper to use for thumb hole adjustments?

[LOUVIT]

I use the smooth black paper which is used for wet sanding auto bodywork etc..and I sand up and down not around the hole. I had had my span widened and I'm still sticking I will sand it tomorrow. It seems all plugs need to be sanded.

[AlexNC]

For minor adjustments I use drywall screen

I second this. I have a workout tool and rasp, but it's easier for me to make small changes without taking out too much material with the drywall screen.