No, Splat, YOU are the one who obviously doesn't know what you're talking about.
Insulting people who do when you ask for help doesn't get you very far.
If you worked on that bore for TWO hours with JB, you grossly overdid it.
There's no specific "hour limit", but the stuff is an abrasive & NOT meant for that much exposure.
Two hours is not "sparingly", the word you're looking for is excessive.
I've used JB for probably close to 20 years.
This is your first time?
And YOU'RE telling ME I don't know what I'M talking about?
I understand what it does, you don't.
The black stuff you scoured off was your bore steel for at least the last 100 minutes.
Next time you ask for help, either LISTEN to what people tell you, or don't bother asking in the first place.
Denis