I have done something similar with gypsum plaster, which fills well and is easy to work with. I used some kind of primer with sand grains or something in it, and plaster mesh. Gypsum becomes chalk-white, so I haven't painted it yet, but when I do, it will be with a paint specifically for plaster that breathes. I avoid plastic components because it gets really hot above the stove, and I have actually got some hairline cracks in the plaster where it gets hottest, but they appeared early on and haven't changed in a couple of years and aren't clearly visible. Tips on how to plaster correctly and neatly can be found on plaster manufacturers' websites.