First, you have to determine what exactly caused the leak, crack, corrosion, etc?
Second, when Halibrand delivered the magnesium wheels they were “pickled” with a protective diechromate process made by DOW chemicals to protect the magnesium’s exposed surfaces from corrosion. This protective coating is the ONLY thing that will protect the magnesium properly. You can paint over the magnesium but only if it has had the DOW process done prior to painting. If you paint directly over the magnesium the wheels will rapidly corrode, sometimes without any clue that it is happening but it is. The corrosion takes place between the paint and the surface of the magnesium where a moisture layer starts. The only way you will catch this corrosion is when it breaks through the paint as a white powdery material but this does not always happen. This is the reason for the common misconception that magnesium wheels rot from the inside out. Magnesium needs oxygen and humidity to start the corrosion process. If the DOW process is left unpainted it will eventually need to be redone as it is only a primer for paint and when left exposed it will deteriorate.
You can
oil the wheels as suggested above but it still leaves the magnesium exposed, even the smallest pores can start the corrosion process. There is a spray penetrant made by Gibbs that penetrates down to a very small hole diameter but it’s still not 100% protected.