In my experience C series cerakote has been very durable. I've used it by request on handguns, optics, Yeti Ramblers, barrels, car and motorcycle parts, knives, etc...
Some say it doesn't hold up as well as the H series variant, but depending on how the item is being used, I haven't seen it wear all that much faster.
The people running around with C series on their hand guns have the same luck with kydex holsters where you can't predict when it'll show wear. I've used it on knives in my shop and then used those knives after full cure to create holes in acetone cans to dry them out, and the cerakote didn't really wear until I had been doing it for about 4 months on cans, boxes, paper, etc..
Now with that said, to address the part about prep. On all of the cases above, except maybe a couple, everything was fully prepped as I would items being done in H series. On that knife I spoke about before there was no prep at all except an acetone cleaning. (Kershaw knife.) One some optics I'll use to steel wool (000 or 0000. Something very fine that you can't really feel major changes, but they do the job without hurting the optic. There there's always the other few odd jobs here and there where you have to adjust your technique for best results.
Long story short, I highly urge you to prep your items, as you'll have better results. If you don't mind doing it again and are limited on prep, you'll find that the cerakote will stick and do its job, but the chance exists that you'll have premature wear. Not a big deal if it's just a quick job that you know you'll do again in the future either.
To touch base on barrels a little more. I air cure them whenever possible because it performs better when heated up. I typically blast them, but there have been the few experiences when I was experimenting and tried either wiping them down with acetone or steel whool or even a green scrub pad. The parts took the cerakote just fine and I've never had a customer come to me with a problem, but I did tell them up front that if they ever have an issue, I'd be happy to redo it. This is normally only done with close friends and family so I can closely monitor the job in the future too.