How do I remove old paint from the plaster before gluing the insulation boards?

Or is it sufficient to 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 attached with a special adhesive. According to the manufacturer, however, the old paint must be removed beforehand.

However, the paint adheres very well to the plaster (except for a few spots). Using the multi-sander is a huge task, creates a lot of dust, and I estimate I will need large amounts of sandpaper. Does anyone have advice? Or can the paint (no idea what type of paint material it is) remain with a good primer after all?

Maybe with a wire brush in an angle grinder where you can adjust the speed? I have seen there are wide flat chisels that you can put in the drill. The guys who removed paint/plaster for me did it with a milling machine. You might be able to rent one at Obi. It's a crappy job.