I liked this solution, re-download the missing keys from the Ubuntu key server.

For Ubuntu

UBUNTU: Fixing GPG Keys in UBUNTU

In my case

Reading package lists... Done

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 2EA8F35793D8809A

Here is the command to add the missing key mentioned in the error message.

sudo apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys 2EA8F35793D8809A

So I take the missing key 2EA8F35793D8809A and ask the keyserver.ubuntu.com to added it to the list of keys I have in apt.

For linux mint

Your key server is slightly different keyserver.linuxmint.com

sudo apt-key adv --recv-key --keyserver keyserver.linuxmint.com "missing key"

Alternative key servers to try

keyserver.ubuntu.com