Bitcoin QT find private key

In order to change the way you keep your wallet, you often need to recover your private key. This key can then be imported in your new wallet or printed (or hand written) on a paper for cold storage.
Just follow the lines and you will be able to manage your private keys as you like...!!! WARNING !!!
When possible, perform those actions offline on a fresh a secure OS. (liveUbuntu for instance)

From Bitcoin-qt

Launch bitcoin-qt. You can do it offline if needed.

select the 'console' tab

walletpassphrase "your walletpassphrase here" 600dumpprivkey [your public key here]walletlock

Once you get the private key, you can copy it now and use it as you wish. Please, ensure you clear your clipboard/history afterwards.

You will need to retrieve your private key for each of your public address. To get this list you can type

listreceivedbyaddress
If you are used to bitcoind, you can use those commands the same way.