This winter, I am going to be getting my bike re-painted, want a different color and have some corrosion that needs attention. It is going to be sprayed with Cerakote. I am also going to be doing a complete drivetrain upgrade in the spring. With this, I will be leaving behind the square taper BB. As a result, from my research, this needs the BB shell chased and faced. Should I have this done before or after paint? If it helps, the coater I use won't have any issue masking off the face for media blast and paint.

Side question related to this upgrade path. If I want to install a 9 speed Tiagra cassette, circa mid 2000s, what am I going to need for spacers on an 11 speed freehub body.

I always do it after.
on a frame I built I also do it before.

Kinda useless doing it before. You'll need a 1.85mm spacer behind the cassette.
Also this kind of question should be 'bicycle mechanics' not GD.

Just thinking out loud, as it were. Could you leave the BB in for the painting, then just unscrew it when you're ready to build the bike back up?

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Brifter: a good bit of jargon to cover generically a combination brake and shift lever, or to acknowledge the common function of Ergo levers, STI levers, and Doubletab Controls.

Always do these things after its been painted/coated because that is when the threads will need to be potentially chased and the head tube and bottom bracket will need to be faced. When paint is on the threads, bolts dont secure easily/well. When paint is on what needs to be parallel and flat surfaces, they arent parallel or flat.
It doesnt hurt to do these things before painting too, but they are presumably already in good condition before painting so they shouldnt really do much.

Im curious what you have on the bike now and what specifically you are upgrading to as 9sp mid 2000s Tiagra is multiple generations old at this point. It works great, one of my kids has a mix of 9sp Tiagra and 105 on her road bike and its dead solid, but its quite old to usually be called an upgrade.

definitely after. any paint on the outer edge of the BB shell and in the threads will interfere with the bit of the BB. facing the shell and chasing the threads will insure a easy, secure, flat fit.

9 speed Tiagra, mid 2000s is the best shifting system I've ever used. Good choice. Newer Tiagra FDs, like 2020, shift better than older; but 2007 Tiagra RD shifting is superb, and the levers and hoods are just the right size and beautifully finished. Also cables that are not under bar tape are easy to change and adjust on the fly in the rare instance you'd need to.

Leaving the BB in the shell isn't an option because the frame needs to be etched before and baked after at 300° F for an hour, so any grease and low temp seals will be destroyed.

The coater I use is exceptional at keeping Cerakote out of places it isn't intended. As sprayed, it is only about 0.0005"-0.001" thick.

I currently have 9 speed Tiagra, but upgrading the wheelset before the drivetrain.

And the shop I use for Cerakote is exceptional at keeping it out of places out isn't intended to be. Plus, it is only 0.001" thick at most.

That isnt thick, but the bb and head tube should still be faced if needed. Do it after the coating and call it a day.

Unless the bike was built incorrectly, facing would only amount to scraping the paint off. Otherwise, it should have been done ,any years ago.

Any bike painter should take care of this as a normal part of the service. If you are using someone who does specialty coatings who doesn’t know bikes expect a world of problems.

With a square taper BB, there isn't any need to be much more than saw cut square. With newer style BB's, the squareness of the face is much more critical.

The person that is doing the painting isn't a bike painter specifically, but he does do a fair number of bikes with both powder coat and Cerakote.

If she is an experienced coater, and knows where paint isn't supposed to be, what sort of problems would there be can't can't be taken care of with a tap, reamer, or facing tool?

Yeah, it is a very thin film thickness. That was/is one of the benefits of it, especially with its original intended use. With most firearm parts, it doesn't mess with tolerances. Their C series, common with coating pistons, is half that thickness.

First thing a bike painter does is a full bare metal inspection. Someone who coats pistols has no idea what to look for. When I get frames painted basic alignment is an included service. Very much doubt that a gun shop would have a frame table. I could go on. Frame painting is a real specialty service. It is not just applying miscellaneous coatings.

I am really confused by this thread. Questions were asked and they were then answered. Those answers seem to be less than acceptable or satisfactory, so they were disagreed with and information has been given to show the initial questions are already known.
Why start the thread in the first place if you have all the answers?

​​​​​​Not a gun shop. She only does powder coat and Cerakote.

most factories don't face and chase bottom brackets. Or maybe they just chase the threads. I just faced a new production frame today that had a lot of distortion of the faces of the bb. It was actually enough to be somewhat annoying. It's pretty typical on production frames to see head tubes that need to be faced as well. In either case, it's almost never just a matter of scraping the paint off. Very few bike shops do this on all new frames, with the thought that the factory does it. Some never do it, and others only do it if they think it's causing a problem.

It isn't that they aren't accepted. I am just starting my experience and knowledge as a base to ask follow up questions to better understand they whys for people's answers.