For small areas, I use drywall compund. For larger areas, I use gauging plaster. Something like Gold Bond Super White. That assumes that the brown coat is intact. If not you need to put that down first and allow about an 1/8" for your finish coat. Plasterers are a dying breed. Finding real plaster can be a challenge sometimes. Since plaster for the past 70 years or more has been gypsum based, they haven't been shaving horses, so your hay burner is safe. :wink: