It is a common problem that the outside metal on the spark plug get rusty after a while when change a new spark plug, especially rain days. Apart from the aluminium anti-seize apply on the thread and dielectric grease on the boot, what recommend to apply on the metal part where it bolt in 5/8.

 

I've never applied anything! The body of the plug is likely to get pretty hot while riding, so anything you use will need to be heat resistant, or it is likely to make the plug look ugly for a different reason.

 

awhile back, champion had issue with this and it went back to an over-seas supplier who made the bases, since then they chose a diff source but if it is of crappy origin, not much you cand do but maybe complain to them or switch brands.