This is a piece of flake board that will be used in a spot not seen but possibly felt so it needed a good feel. Also it needed paint but without sucking up a lot of it. I troweled on a coat of wallboard plaster and gave it a light sanding. It took paint well and is decent in one coat. It should be fine.
Plaster filler.jpg
This seems like a reasonable method to improve shop walls if you have them. A second coat of plaster after sanding would be better and with a second coat of paint it should look really good.