Hey Guest!
What have you got against rosin core?
Only trick I know is the same as for electrical stuff; the surfaces have to be clean and free from dirt, oil, oxide, etc. Acid flux is better at cutting through oxide than rosin, but that's the only difference IMO.I learned rosin for electrics, acid for mechanicals. In a pinch, I have tried rosin on mechanicals, but could not get it to bond...have you found a trick?