Offline Primary Key

[Phillip Susi]

I would like to keep the private portion of my primary key stored 
offline and use an expiring secondary key for day to day signing.  To 
accomplish this I have tried backing up the key after creating the 
secondary signing key, then attempting to delete the private portion of 
the primary key from the key ring, but even when I explicitly specify 
the primary key ID to delete with --delete-primary-keys, the secondary 
private key is also removed.

How can I remove ONLY the private part of the primary key, and not the 
secondary key(s)?