I built my tandem with powdered flux. There may still be some under the paint (j/k, I hope).
I built 10ish frames with that flux, although mostly lugged with silver, so that would just be the bridges and dropouts. A number of those were after Trek, so it was even more painful because I was used to the chemical treatment system they had. I like having some powdered flux around, even though I almost never use it.