FOR YOUR LEISURELY PERUSAL
THE IDIOT WITH ACCESS TO A GARAGE PRESENTS
How to Fix a Dented Filter Ring on a Lens
FOR THOSE WITH A RATHER LIMITED FAITH IN THEIR WALLET
AND AN OVER-WHELMING DESIRE FOR HITTING THINGS REPEATEDLY
Oi oi! How goes it? 
A little while ago I was a complete twat and pranged my 24-70L. I don't even know how I managed it, but I had dented the outer portion of the lens filter thread.
"Gosh-darnit," I said in my best Deep South accent learnt from watching the American documentaries True Blood and Deadwood, "I'm not going to spend my easily-spent money on repairing this expensive thingy!"
And thus I set about finding a hammer.
Exhibit #1:
First off, I found some soft wood. I'm not very scientific, so I can't tell you what sort it is - all I know is that I was able to mark it purely by pressing a thumbnail against it. The relevance of this is so that when you place the wood against the filter thread and apply pressure that it will [hopefully] mould against the metal thread instead of misshaping it.
Ideally, this soft wood needs to be circular in shape, with at least one flat side so as to be flush against the front element. I don't have much patience for inanimate things, so I used whatever I could find; if you're meticulous/sensible, cutting a circle with a diameter slightly smaller than the filter thread would be perfect to retain shape.
To add some semblance of protection, I found a soft, cheap dish cloth to place the lens on while it sat on the work-bench. Furthermore, I placed a lens cloth between the soft wood and the filter thread, just for a tiny bit of extra shock absorption and to stop any rough wood rubbing against the front element of the lens. You may wish to use more protective materials such as thick cloth or polystyrene.
Exhibit #2:
With the basic set-up ready, I gave the wood a few taps and determined it was working, albeit very slowly, before realising I was using my hammer (which was a fairly tiny one) a bit too close to the front element. To combat this (and to allow me to hit it harder without risking smashing the glass), I found another wedge of wood (hardness irrelevant, as it was to be placed on the existing wood and not the lens), as shown below.
Exhibit #3:
Repeat. And repeat again.
After every half-dozen hits, I'd take the lens away from the bench to look at it. From here it really is all trial and error; I found towards the end that I was risking bending a portion of the thread *outwards*, so I stopped.
With a small application of vaseline, I gently managed to get a 77mm CPL to bite on the thread. Joy! The first time around I got about half a full turn, then undid it a bit, and tried again. At the time of writing, my CPL sits securely on my 24-70L. Not flush, but secure, and able to be removed without any difficulty.
Exhibit #4:
While a lot of this may make some of you DIY buffs smirk or turn your noses up, I hope this tiny bit of stupidity and elbow grease will help someone else avoid a sizeable repair bill!
I'm off to the Algarve for two week's holiday and I fully intend to enjoy using my new CPL on my 24-70L.
Happy shooting!
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