If it is a grinder as it appears 400 is way too fine. A grinder is for taking lots of material off quickly so 24 or 36 grit is what I'd recommend. A DA or orbital palm sander would use 400 grit OK.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.