the valve covers are a magnesium alloy, not an aluminum alloy. magnesium corrodes from water. the blistering shown in your picture will worsen until it eats all the way through. we are seeing it starting on most crossfires/mercedes now, there is no repairing it. you can see if it will outlast the vehicle or replace the cover with a new one before they are discontinued. that will eventually be the death of these engines. we started seeing this about 10 years ago, at which time we stopped refinishing used valve covers as all the used ones came in with pitting/blistering. i have a pile of dead valve covers here all with the same issue.