It is very common to top [skull, hit thin] long iron and fairway wood shots.
Let’s start by breaking down the cause and effect behind topped and sculled shots so the “cure” videos herein make sense.
When we see the steep loft on the club face and the un-teed ball laying flat on the ground, it is only natural to think we have to help the ball get up into the air by trying to use the club face as a scoop.
> This visual effect and the associated logical [though misguided] idea that we need to scoop the ball up into the air causes a scooping action with the hands where we lean the club shaft backward [opposite direction of the target] into-through impact.
> This backward shaft lean in an effort to scoop the ball up off of the ground is created by cupping the back of the left wrist and bending the left elbow [and flexing the right wrist].
> That cupping and bending action shortens the natural arc of the clubhead so that the club face can’t reach the ball.
> That cupping-bending action also tilts the club face away from the ball thereby exposing the sole’s leading edge to the ball.
> The effect from the above factors is that the sharp leading edge contacts the ball [versus contact with the club face] at or above the equator of the ball.
> Because that sharp edge is headed upward it imparts a topspin or knuckleball spin.
So let’s start with the correct, albeit counterintuitive, understanding of the desired interaction between the club face and the ball at impact;
>  While keeping in mind that for a long iron or fairway wood that we move the ball forward in our stance so we have more of a glancing downward blow on the ball,
– versus a more definite downward blow for shorter clubs where the ball is positioned more toward the middle of the stance.

“Discovering And Embracing The Realities Of Impact To Replace The Exact Opposite Of What The Mind Believes” [Article w/ embedded 2-minute video]

“How the ball flies” [2 minutes]

Now let’s get to the cure for hitting thin shots by understanding that these shots are caused by a too-steep attack angle into impact;
> If the clubhead were a plane, that means it is nose-diving toward the ground and destined to hit the ground before hitting the ball,
– UNLESS you abruptly pull it up out of the nosedive – which is what you are doing but in a way that overcompensates for the nosediving action with a quick upward scooping action thus causing the thin/topped shots as explained above.
> …versus the plane/clubhead coming in for a smooth landing [contacting the ball before touching the ground] only to smoothly push back off of the ground back up into the air.
A too-steep/nose-dive attack angle is caused by the all-too-common over-the-top move, otherwise known as an outside-to-inside swing path.
Here are three great videos from my mentor, Peter Croker, PGA-Australia that will concisely and directly explain and demonstrate the cause-effect and how to fix it:
“Golf – Curing the Slice. Presented by Peter Croker”
“How to Fix the Over the Top Shot”
“The A to B Drill To Ensure the Club is on the Right Swing Path”
At some point you’ll have to do the opposite of leaning the shaft backward at-through impact and instead lean it forward;
> While keeping in mind that on longer clubs there isn’t a lot of forward shaft lean because the ball is positioned more forward in the stance,
– as compared to using shorter clubs where the ball is back in the stance and the shaft lean is therefore more exaggerated.
In order to do the exact opposite of what you have been inclined to do for many years, some exaggeration of that opposite movement can really speed up your results.
So, here are more detailed demonstrations along with some drills hitting smaller shots with shorter clubs:

“Physiology of Impact – Chip Shot Technique Demo” [Article with embedded video]

Video – [Also embedded in the above article]:

Video – Maximizing Mass Transfer From Body To Clubhead At Impact [5:02 to 9:40]

Video of Ted demonstrating chip shots [1:02 to 2:55]

“Chipping Around the Green for Maximum Backspin / Croker Golf System” [3:20]

Now, in order to compress the ball as described in the above videos, we need to understand the “secret” of the late-hit action that we see in good ball strikers.

“Dispelling the illusion of the late hit angle / Croker Golf System” [2:27]

I hope you find the above cause and effect breakdown and the videos on how to overcome the over-the-top swing [the root cause for thin shots…as well as for fat shots and sliced shots and pulled shots and pull-hooks] helpful.
Feel free to email me with any questions or comments.
Ted Williams
Certified Instructor – Croker Golf System
Head Coach/Featured Contributor for My Golfing Store