What finish was used for English iron cannon barrels?

[Tar Bucket Bill]

Were English cast iron cannon barrels of the GOAP period [and later] painted with black oil-based paint, blackened, or Japanned, or something else entirely?

My next question would be about what the mixture was for any of these coatings.

A friend of mine wishes to duplicate the proper finish on a Traditions .69 caliber naval cannon barrel. I think right now he's just leaning toward the black oil-based paint finish, whatever that proper mixture would be.

I haven't really found any sources other than possibly coated with a black oil-based paint, or blackened. That's kind of vague, since no details to their proper mixtures or procedures were given. Not sure what "blackening" entails on a large cannon barrel, unless that means heating the barrel super hot and then dipping in oil.

Thanks for the input.

-Tar Bucket Bill