[Closed] Repair a carbon frame

Is there any evidence of the crack in the carbon now? If you tap on it with a penny or similar does the sound chance.

Right now it looks like you've sanded about 6 layers of paint and just a couple of layers or carbon. The primer is the glossiest of the black layers before the more graphite carbon layers start. You have not gone very deep at all.
If the 'crack' has gone, I probably wouldn't even worry about a repair considering the minimal material you've removed as it might just have been paint damage. If the crack is still evident, keep sanding until it's sanded out completely.

For repair, I'd recommend the easy composites resin and hardener from their fishing rod repair kit (available separately) and a bunch of small pieces of sample fabric along with shrink tape. Cups, brushes and stirrer sticks from your local pound shop.
I don't think that warrants a full wrap of carbon around the tube as it looks pretty superficial to me. At no point would I want to repair a carbon frame so it looks like a repair. Full wraps of resin/fabrics...erg. if this is needed, if want to be sanding out layers everywhere new fabric is going and then ultimately sand back the overbuild to match the original tube profile.
With yours, as long as the evidence of a crack is gone, I'd just laminate in a couple of layers of 'sample' fabric and sand smooth.

I am not a carbon engineer or expert.

I posted a carbon fibre frame repair on the Trigger thread a couple of weeks ago, link;

Carbon Fibre Repair

I did use an EasyComposites kit but I went with the fishing rod kit as the carbon fibre cloth is thinner so I could use more layers, it also included shrink wrap. I filed away enough paint and carbon from the frame to lay three layers of new cloth. The kit also had some polish the repair looked great after and I didn't feel the need to fill and paint.

Great. Thanks fooman and Bearback. Looks like I will go with that. I got a full bike for the equivalent of £320. Just needs a new cassette, chain and carbon patching.

The original 'crack' just looks superficial to me (like a paint chip in a metal frame) , if it was my frame i would just ride it as usual without attempting any kind of repair or giving it a second thought.

It just needs a sticker!

Did the person selling it tell you how they did it?

I. E it was a massive fall and hit a huge rock or they were cleaning it one day and just found the mark and stopped using it?

Do it, you've nothing to lose by trying.
Here's my frame repair thread from 5 years ago, still going strong... I link it every time this topic comes up now.

Having done the shrink tape compression method, it works, but if you can find a way to vac' bag it or at least apply more even compression you run a better chance of getting a decent well consolidated repair

Given the location I'd strip the paint off and do a local "splint" around the full diameter of the tube. Ideally tied in with a wrap going round the back of the seat tube to sort of cross brace the joint a couple of layers of 200g woven fabric (widely available as a rolled 'tape' type product) and then maybe wrap over the whole repair and the tube junction area with some tow for extra belt and braces (probably not necessary TBH)...

Can you get at the inside via the seat tube at all?

Just did this on a frame. there was a small paint crack that may have been nothing but I'm stripping the frame for custom paint so didnt want something transmitting through into the new paint if it was in the structure. So, I sanded through 2-3 layers of carbon to create a 'trough' then shrink wrapped in 1 x UD and 1 x 90gsm 2k weave then sanded them flush. Any imperfections I'll fill with spot putty.

I’ve got a lovely super light Scott frame with a crack all the way round a chain stay right next to the cable stop.
I can’t see how I’d get to it to repair.

@jkomo
If its cracked but not separated then maybe you'll still have enough unbroken fibres to maintain its geometry/relative position?
What about sanding off the cablestop, sanding out the crack then doing a full wrap repair.
Glue a new cablestop on afterwards or use full length outer.
This was a friends crash damaged seatstay, and the first repair I got into. It wasn't broken all the way round so even with the bulk of the cracked fibres sanded out it still had enough stability to stay in shape during repair.
this first photo was at tackup stage of the epoxy, strands I figured might help support the actual repair fabric..
I sanded all around the seatstay to give it a depression to work the new layers flush and the overlap was done where the bulk of the damaged carbon removal was done. Then 2 extra UD layers over top then sanded flush to the original.

Carbon dust and respirator use aside, its quite stress relieving doing it yourself.

I’ve got a lovely super light Scott frame with a crack all the way round a chain stay right next to the cable stop.
I can’t see how I’d get to it to repair.

I assume you mean an external rather than internal cable stop, in either case Bearback's advice above sounds sensible to me.
Got any pictures?