Don't know if this helps, but when I was in my gun nut phase, and having particularly acidic skin residue, I used to spray my cap and balls down with carb cleaner after cleaning and then use a heavy gun oil (I forget the name) to wipe them down with. After a long boat trip (45 days or so) my firearms would be rust free, even in the 80-90% humidity we have here.
I use a similar technique with my tools now when I finish a project, but, like you said, with wd40 and have had no problems. Just like with me, it might be a skin issue.
"If the doors of perception were cleansed everything would appear to man as it is, infinite."