Iggy,
This depends on the investment you want to make. If it's a quick and dirty, go to Brownells and get a small bottle of gun black.
Of course it looks like it is, a patch job. If you want to do it right, you must repolish to white metal and reblue in a salt bath
at 295 F. The blueing salt can also be purchased from Brownells. If you do not return to white metal, you will get blotching. If
you want to do the latter, read a book, it can be tricky, but not difficult. I used to do it a lot, but I no longer maintain a
salt bath. the key is clean, clean and very, very clean. Not even skin oil. So we are talking about white cotton gloves.
Steve

"Ignoramus9556" wrote in message ...
I have a little 20 gauge shotgun that has some rust on the barrel. It
is functional despite that, but I would like to clean it up a bit. I
would like to know what is the proper procedure. I guess I can remove
rust and even polish it a bit, but then I would somehow blacken the
barrel again. Is there anything that you would recommend for this sort
of thing. Thanks

i