“no public key available” while upgrading using update-manager

refs:
http://askubuntu.com/questions/520718/no-public-key-available-while-upgrading-using-update-manager
This warning means that you don't have the corresponding public keys for these repositories.

W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,
W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,

Verify that these keys are indeed the ones you need. In this case:

➜  ~ gpg --keyserver hkp://keyserver.ubuntu.com --search-key 3B4FE6ACC0B21F32
gpg: searching for "3B4FE6ACC0B21F32" from hkp server keyserver.ubuntu.com
(1) Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>
4096 bit RSA key C0B21F32, created: 2012-05-11
Keys 1-1 of 1 for "3B4FE6ACC0B21F32".  Enter number(s), N)ext, or Q)uit >

The ID correspond to Ubuntu Archive. It is strange that you are missing these keys. This is normally fixed with:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

After this, run

sudo apt-get update

and make sure no other warnings/errors appear.