I have a Sharp microwave oven which is a combination oven as well which I use quite a bit. It has become quite marked on the inside and I am wondering what to use on it to get the marks off. Can I use a conventional oven cleaner as this is just used as an ordinary oven or would it ruin the surface of the oven when I want to microwave? I have tried all the tips of using bicarbonate of soda etc., but wonder if I use anything stronger which would shift the stains and get it looking brand new again.