Hi boyracer, the cover you efer to is the waveguide cover, and as such is very important to the healthy life of the oven. arcing is food deposits burning up into carbon and then becoming microwave conductors. No time for lessons here. Clean the food off gently or buy a new waveguide cover. If you can't find one by a peice of sheet material and make one. You must also remove the "smoke" marks they too will have carbon content. Humbrol model enamel, the little tin type will do for paint, there are no high temperatures inside microwave cavities. Find me on www.microwave-repairs.co.uk
kindrest regards
patrick, March 2010