Does it work well to use Board/Masonite as wall panels? Today I have some type of wall panel with milled grooves that I want to cover to be able to wallpaper or paint. Gypsum 6mm feels like it builds too much and complicates things with windows etc.
The problem with masonite is that it tends to swell after a while due to humidity, which makes it warped. I once heard a tip that before you put up the masonite, you should dampen it so it swells, then install it and let it dry. Apparently, the effect is that it contracts and stays smooth in the future. Whether this is true I don't know, as I haven't tried it myself, but maybe someone else on the forum has done it and can provide some insight if it works.
I think you should completely ignore doing anything with these masonite sheets other than tearing them down. How on earth did they think when they put them up??? I have a house from 1972 and have these sheets on the wall in the living room. When I bought the house about a year ago, it had an orange textured wallpaper, so I took a chance, as it always is, to paint over wallpaper... It went well, after some plastering, sanding, and painting, I was incredibly satisfied until they started to swell... Now that I'm selling, I've spent the whole darn evening tearing down that stuff and tomorrow I'll buy drywall and put it up instead. My tip is to tear it down; you'll regret it otherwise.