If the part is too warm, the soln will plate out the copper easily too.
'Too warm' in my experience is anything above 75F or so.
If you are Rust Bluing,,let the parts cool off well after taking them from the boiling water and and carding.
Add to that any aggressive and repeated passes with the soln applicator and you will get the copper plating.
Sometimes copper areas rust OK,,and sometimes it does seems to resist rusting .
If it does the latter, I usually just scrub the copper plated area down with some Scotchbrite and blend it into the surrounding areas.
Then recoat everything and continue on.
If it rusts OK, the coatings will catch up with each other and all will be well.
No need to polish everything and start all over again.