Deal with W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6AF0E940624A220
There you can enter your key into the Search String field, leaving everything else as is, then click "Search!". When searching for a key, always prepend 0x to it! This means that, for example, you have to search for 0x6AF0E1940624A220 instead of 6AF0E1940624A220.
The page's heading should be similar to Public Key Server -- Get "0x6AF0E1940624A220" Copy everything from underneath the title (starting from -----BEGIN PGP PUBLIC KEY BLOCK-----) and save it in a file (e.g. key1).
Once you have the file, run the following command
sudo apt-key add key1
You will get an "OK" response.
And you are done. Repeat the procedure of other keys that might be missing.