If I paint or Duracoat a barrel, what do I need to do around the gas port?  Just block the hole so no paint gets in, or do I need to leave a larger area around it unpainted?  I'm going to be using a YHM gasblock with flip up sight.

I use a rubber plug for the gasshole and the muzzle and chamber end

Take my word for it.  Do not paint any area the FSB or gas block will be in contact with, unless it is place, or taped off.   Or your next post will be how do I....
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I wouldn't worry about the hole too much, but a small plug would be fine. I have done 2 I think, where I didn't plug the hole at all. (Might have only been one.)

Like the last poster said, you might run into trouble with painting with the FSB off. I don't know what specifically he is referring to, but there is a very good chance you will scratch the barrel when putting the FSB back on. You might put a few light coats on, let them dry for a couple of hours, then assemble, and coat again.

As long as the barrel isn't in the white, I'd assemble it and paint it that way.

Well, here's the gas block I'm using:



The top and bottom are separate, so hopefully I wouldn't have to worry about sliding the gas block over the barrel and scratching it.