gpg --export-secret-keys -a 1234ABCD > secret.asc
# or
gpg --export-secret-keys keyIDNumber > exportedKeyFilename.asc
gpg --edit-key (keyIDNumber)
gpg> trust
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
1 = I don't know or won't say
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
m = back to the main menu
gpg --import mypub_key
gpg --allow-secret-key-import --import myprv_key