I just got a new F500 Oslo which looks pretty good, but there is a pretty extensive amount of surface rust all over the inside of the top plate. I'm thinking about replacing with a cook plate anyway, so I wanted to see how difficult this is to take out, and was kind of alarmed to see rust covering the entire bottom of the plate. Is this normal? Should I be worried about it spreading to the body of the stove or am I just being silly?