Quoted:
plug the gas block?
eta: if you have an original chrome-lined barrel, you don't need to worry about plugging anything. the phosphate won't react with the chrome at all. i've built easily two dozen AK's over the years and haven't plugged any that were parked.
if you're using a us-made chrome-moly barrel, then yes you DEFINITELY need to plug that up.
No, you definitely don't. I used to think this way as well, "If it will do that to the outside it will do that to the inside" but it's not true. Luckily Gunplumber was gracious enough to impart some of his knowledge to me and I tested it on a few US made non-chrome lined barrels and it had no effect on the bores. Now if you have a barrel that's already a bit frosted then yes, that will give the park a place to bite down on, but it won't on a clean polished surface.