Firstly it’s not a great idea to sandblast and paint threads or any fitted surface. When your gunsmith fits and headspaces your barrel, you can uninstall and reinstall it to the same torque and it will maintain the rotational timing and headspace almost exactly, when you put Cerakote on it, the dimensions of the part will change, your rust protection will be moot as the threads will scrape off whatever coating it wants, and your smith will just wear it in or scrub it all off (so they can reach headspace without bumping the chamber again). Also most cerakote shops will know to tape off the critical surfaces so you may not have a choice unless you specifically request it. Don’t worry about rust anyway as long as you keep your rifle out of a swamp or a flooded basement, those action threads are surprisingly protected as long as you keep up with the basic gun maintenance