Note: This video is not about whether traitors can be redeemed, but rather if the physical and spiritual corruption caused by Chaos can itself be undone. For a video detailing the potential redemption of traitors, click here:
https://youtu.be/lzKWRl1Acfs

If Chaotic corruption can't be cured, there's always a bolt shell to the face

Vork briefly becomes his uncorrupted self during teleportation
Chief Miles O'Brien: Sorry about the transporter malfunction.

There was one case of someone whose Chaos corruption had been undone and it was Tornus the Redeemed.

He was once known as Torglugg the Despised , a Chaos Champion of Nurgle but a small part of his noble spirit remained deep within the corruption and Celestant-Prime smashed him with Ghal Maraz to set it free, allowing him to be reforged into a new member of the Stormcast Eternals.

I would also argue that Yvraine proved you could cure the physical blight of chaos when she reversed the Rubric of Ahrimann. As the revived marines didn't begin falling to mutation and spawndom, it can be assumed that their physical bodies were at least temporarily stable. This physical state may have been temporary from the beginning, as it appears Yvraine was merely using it as a short-term bargaining chip and had them killed to prevent Ahriman from discovering her ruse.

I've always loved 40k Lore, never quite got into gaming but your channel amongst the several other awesome 40k Channels are my absolute favourites. Thank you for these wonderful videos.

I remember there were 2 extra instances where this was achieved somewhat:

1) I remember a story where there was Plague Marine and a Sister of Battle. SoB was friend with the astartes before his downfall, so she went to the Eye of Terror or something to redeem him. He eventually redeems himsel by suicide bombing himself with the Daemon Prince he was serving to.

2) I also remember a story where a group chaos space marines (Nurgle Plague Marines always for some reason) where they got… freed from chaos corruption for a brief moment. Then, when they looked each other, they felt extreme disgust and shame with themselves. Can’t remember fully but it should be something like this

Just have a sensei punch the corruption right out of them, redeeming even the most corrupt chaos warband leaders into loyal allies with the power of friendship.

"If it is redemption you seek, I shall give it to you..."  *bolter shot to face*  
~ Brother Captain Gabriel Angelos

I think it is a moor point, whether Imperium or any faction no one would ever trust a "cleansed" servant of Chaos. There would always be doubt, blame, mistrust, and the very real possibility of re-corruption.

I think it’s worth looking into the book tech priest dominus. The main tech priest Omnid Torquora undoes chaotic corruption from one of his captured servants by holding him next to a gellar bomb and letting the gellar waves wash over him

4:48 There was a Loyalist clone of Fulgrim being pure uncorrupted DNA and he is currently at Trazyn the Infinite's collection.

It would have been interesting to see Loyalist Fulgrim Clone atone for his sins by fighting for the Imperium and lead his Chapter of Loyalist Emperor's Children, like the Sons of the Phoenix that looked and acted like the Pre-Heresy Third Legion.

Welcome back Inquisitor!

By the Gods! Welcome!

Answer:
Only GW's power of random bullsh*t alone can do this.

This is by far the best subject you have ever thought of

Just read, "Author Interview – Ben Counter" from the thebolthole:

Arguleon Veq from Daemon World is one of the most loved villains, some would say hero, in the setting. How did his tale start?
Ben Counter: Veq is there to demonstrate that Chaos is a lie. Chaos is a wonderful adversarial force because it can be anything, and it only really has one rule – if you give in to it, you will be corrupted, and become something awful. Bowing to Chaos is never, ever worth it, no matter what you might get out of it. Veq is one of the few people who went too far, realised it, and managed to drag themselves out of it. That was the core of the character, someone who could illustrate something fundamental about what Chaos is and how it works.

He was almost a deamon prince and what Veq did with the Maelstrom wasn't a small thing. By the end of the story he seems to have undone quiet a lot of what chaos has done to him.

I seem to recall a handful of Rubric Marines being fully restored before being cast into the webway...

It seems like nearly everything is able to be healed. Even the black rage as shown by Mephiston

The idea of the "nice" gods of the warp being willing to cure the taint of chaos is a nice one, however there are just so very few left due to the wars of the Chaos Gods. Perhaps the Laughing God could be bargained with if...you laugh at his jokes or make a scarfice that he wants.

It's been too long!!.. thank you for all of your brilliant contributions!!

Well, when possession is exorcised the hosts mutations  disappear. I have no idea how it would work on just random chaos corruption.