Re: About to start a build, quick question
The point of that is to ensure that the rear mount is perfectly flush with the seat tube. If that mount is too tight, file or dremel it to fit. If it's too loose, yes, you can build it up with JB Weld or the like, then file to fit.
But the aim is a flush fit. If you already have a flush fit, adding epoxy isn't going to help anything, and may make it worse.