Hi everyone!

I've got a Columbus tapered head-tube (code-name CYRT18250) with braze-on cups (ZCALT46UP, ZCALT56DOWN) for a mountain bike project, and have a couple of questions.

First the headtube instead of being nice and shiny is covered in some black stuff. Anyone know what that is or the preferred method of cleaning it off?

Second, after cutting it to the right length should I face it and solder the cups in before building the frame, or is it better to do that afterwards? I'm thinking before because if anything it will reduce distortion from welding. If things do distort much it's not going to be perfectly round either way and it's just going to make it harder to get the cups in if I do it afterwards.

Any tips for the best way to solder the cups in also appreciated :) I don't think strength is very critical, it's just to stop them moving around the same as a normal press-fit would.