I can't comment on the temperature issue, I always apply the solution to cool metal as I find it easier to get an even coating if it isn't drying too quickly but everyone to their own..!
I have only once had a serious problem with copper plating from rusting solutions and indeed that was a copper sulphate based solution used on items of gun furniture when I was experimenting with alternatives to hot caustic blacking. It was so bad I had to resort to chemical methods of stripping it off where there was engraving.
Since then I have often used it on both steel and damascus barrels with no serious problems. One often gets a thin copper wash on the surface but it still rusts just fine and rubs off with the carding.
I agree with others' comments: don't re-coat, work on cool surfaces and be patient while it dries. If all fails use a non copper solution on this particular set of tubes. It may work fine on others. Remember, this is a 'black art'!
