If you try to decrypt a file using an external keyring as per the instructions here www.chrisnewland.com/av/37/decrypting-gpg-file-using-an-external-keyring and you get the following error:
gpg: [don't know]: invalid packet (ctb=2d)
gpg: [don't know]: invalid packet (ctb=2d)
gpg: keydb_search failed: invalid packet
gpg: encrypted with RSA key, ID ********
gpg: decryption failed: secret key not available
Then you have probably exported your public/secret keyrings as ASCII armoured which is not compatible with decryption using an external keyring.
Simply re-export your keys without
and you can use them for decryption without importing them.