Help with the "unusable public key" message

[W Lewis]

I had a similar problem, which I solved by deleting the non-selfsigned UID
from the key I was trying to use. (I can always re-import that key if I
want that UID back.) This was especially perplexing as I wasn't referring
to the key by its UID --- I was referring to it by its hex key-ID. It
seems that if a key has any non-self-signed IDs attached to it, gnupg will
mark the entire key as unusable and refuse to perform any operations using
it, even if you use --allow-non-selfsigned-uid. I was using gnupg-1.0.6,
and it was an old RSA key generated by pgp2.6.2.