Adding new PPA is causing GPG error after apt-get update

Recently I had a lot of GPG errors with the following message

W: GPG error: http://ppa.launchpad.net trusty Release: The following
signatures couldn't be verified because the public key is not
available: NO_PUBKEY *****************

I removed them with using What is the easiest way to resolve apt-get BADSIG GPG errors?

and with the help from other sites.
But now every time I add new PPA and try to update I got the GPG Error for the last added PPA.

Using Y-PPA > Advanced > Try to import all missing GPG keys removes the error/s but it will show again for every new PPA added.
Can someone tell me why I got GPG error every time and how to fix it permanently.

using Ubuntu 14.04 x64
EDIT:

Problem solved. I was missing /etc/apt/trusted.gpg.d

The problem is that you have too many ppas.

Apt can only handle 40 keys in /etc/apt/trusted.gpg.d.

Delete some gpg keys and ppas.

See here for some more info.

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EDIT

Because you have no /etc/apt/trusted.gpg.d directory, just run the following command to fix the issue:

sudo mkdir /etc/apt/trusted.gpg.d