Interesting though that some discs rust more quickly than others. The Xtrail discs were covered in rust on the braking surface after overnight dew in summer. Left for a week in the rain the initial braking was interesting. The P38 on the other hand shows little rust on the braking surfaces in the same conditions.
There are several different make ups of Grey Iron Keith. Also it depends on what was in the rain. It's not as pure as we may think.