If you ever get an error like this.
W: GPG error: http://non-us.debian.org stable/non-US Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D
W: You may want to run apt-get update to correct these problems
The way I solved it was as follows. As root…
# apt-get install gnupg
# gpg –keyserver wwwkeys.eu.pgp.net –recv-keys F1D53D8C4F368D5D
# gpg –armor –export F1D53D8C4F368D5D | apt-key add –
# gpg –export F1D53D8C4F368D5D >> /etc/apt/trusted.gpg
# gpg –list-keys