Does anyone know what these spots are on our fire pit and how I can remove them? It is a propane fire pit and we always keep it covered when raining (actually we keep it covered whenever it’s not in use). I also have it so the cover it tented that way water doesn’t pool. I tried to clean it with a sponge but it did nothing. You can see in one photo a long scratch from my nail so I think it must be able to come off somehow? I don’t want to scrub it too hard though and the scrubby on our sponge did not work.