When you type in the apt-get update and get the NO_PUBKEY error then make sure that you copy the error number mentioned after the word NO_PUBKEY.
Once you copied that number you can use it along with following command to get gpg key from launchpad to fix that.
For example,
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 61E5F6C15F6C1
Do execute the following command,
sudo-apt get update
as many times as the error occurs and then go ahead with the previous steps to import the GPG key. Once you get the errors resolved by importing the error key, you can do normal apt-get update.
Thats it, after importing the GPG for each error key, you can get the solution.
