It's probably a rasp to knock off the errant gypsum high spots so the
boards will fit more tightly together. Some people use a Surform
block plane tool, some use a expanded metal mesh wrapped block of
wood, and other people just use their drywall knife.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.