Even racks or loads under 50kg will cause hairline cracks in the gutter paint. I've had three Defenders, all with expedition full length racks and they all featured minor cracks in the gutter paint. The hairline cracks are inevitable and should'nt cause any serious corrosion.
The gutters are actually very strong and can take a lot more weight than stated in the manual. Most Defenders in Africa are kitted to a certain standard. You seldom see them without expedition roof racks with tents, jerry's and other **** on the roof. Not something I would recommend but if there were issues with gutters not being able to handle those loads ... people would simply stop loading them to the rafters.
https://www.aulro.com/afvb/images/im...008/10/108.jpg