Don't use "sandpaper" to clean contacts. Its way way too abrasive & will rip any plating off in a heartbeat!
An ink eraser on a ballpoint pen is as harsh as you'll ever want to go. Also a thin smear of petroleum jelly (Vaselene) will protect previously damaged contacts while allowing electricity from the battery to pass through.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”