I've been working for a long time to get my club face closed at impact. If I swing naturally I can have a nice, powerful swing with a good club path at impact and crisp, consistent contact. But with that natural swing I leave the club face open with long irons and woods all the time, and everything longer than my SW on some days. I've been working really hard at trying to finally fix this. One thing that helped was trying a new grip. Just gripping it the way I was but stronger never worked for me, but I happened upon these two videos a while ago and the new grip definitely helped. I've arrived at a grip sort of in between what I interpret as the recommendations of these two vids.
The key differences for me were getting away from having the the club too high in the palm towards the pinky side of the left hand (ie, more fingers, less palm), and the recommendation in the first video about how to start gripping the club with the fingers and with the wrist straight, and then roll the hand over onto the club, rather than sort of clamping straight down trying to keep the wrist straight as you close the grip with the left hand.
Even bigger for me was realizing that my problem was that a natural swing for me was to essentially bring the club face back to the same position it was in at setup relative to my secondary upper spine angle or my shoulder angle. But if you get any decent amount of secondary axis tilt at impact (as one is supposed to) then a face angle the same as it was at impact relative to your upper upper body is open. Try this:
Set up with a known target, preferably something at somewhat close to an actual shot distance, not the wall across the room. Then just statically to a strong impact position, with the hips bumped and well rotated but not pushed towards the ball, front leg straightened, hands a bit in front of the ball, head not moved front or back, etc. Now look at your target. For me at least, with good secondary axis tilt but the head stable, the target now looks like it's almost behind me, which is weird given I'm more facing the target now than at address.
Trying to figure out how to naturally and powerfully get to impact hitting the ball much further left than feels natural now is what I spend 100% of my golf practice time on these days. Going to that static impact position and not getting the club face square just manipulating it but trying to feel through the body what has to be strong and rotated/bumped enough to let the face get square without too much arm/hand manipulation has helped. But frankly, I'm also actively extending/rolling my right forearm and wrists right now, sort of forcing it into position to try to get used to it, under the theory that I can slowly ingrain what the rest of my body needs to do to decrease then eliminate that active extension/closing with the hands.