ᐅ How do I remove old paint from the plaster before attaching the insulation panels?

Or is it enough to just remove loose paint or plaster? We want to cover a wall in the garage with calcium silicate boards (due to high humidity). These will be installed using a special adhesive. According to the manufacturer, however, the old paint must be removed beforehand.

The paint adheres very well to the plaster (except for a few spots). Using a multi-sander is a huge effort, creates a lot of dust, and I estimate it will require large amounts of sandpaper. Does anyone have advice? Or can the paint (I have no idea what type of paint it is) remain if a proper primer is applied?

Maybe try a wire brush attachment on an angle grinder where you can adjust the speed? I’ve seen wide flat chisels that you can use with a drill. The guys who removed paint and plaster at my place used a grinder. You might be able to rent one at a hardware store like Obi. It’s really tough work.