i have a real problem..

[spankit]

...with a closed clubface on the backswing at halfway back position.I start with a left shoulder turn and the only thing that really helps me open it (square it) halfway back is extending the left arm close to the ground as i go back.If i dont get this done it will be shut and of course its shut as i sweep into the ball resulting in a hook.I cock my wrist about the right foot but i still will shut the clubface if im not really careful and then sometimes when i am careful!!Does anyone else have this problem?Its something i constantly have to be on the lookout for.Any suggestions welcome.