I learned this one the hard way. I had some rust on a barrel and had been using stuff called Krud Kutter to remove/convert rust on my truck. I figured it worked well enough, ill use it on a barrel. Well, it didnt really do much to the rust, but it literally "Washed" the bluing completely off that part of the barrel, it took about a millisecond to completely remove the bluing...

I found this stuff either at Ace or Napa, cant remember which. So for you refinishing guys, if you want a quick, easy no mess bluing removal tool get this stuff.

Off to send barrel away to get re-blued. oi vay

Evapo-Rust will remove bluing too.

that sucks.  sorry you found out the hard way, but thanks for passing that along to the rest of us