my first job in high school (late 70's) was at a pcb assembly house. a very small company that had a wave solder line and they made boards for gas station pumps (the digital displays). I had been soldering a few years already, since I started as a very young kid, but I was surprised to see them use ACID flux (I remember it being pink or reddish and very low viscosity).
we would paint a lot of the flux on the board, the board would be sent thru the wave solder machine (it was all open back then, no cage around it like you'd see in today's big pcb lines) and once it cooled, we'd would work at little kitchen-sink areas, running the whole soldered board under hot running water, scrubbing with a brush and using some kind of cleaning fluid to remove as much of the remaining flux as we could. we knew it was acid and could not stay on the board, but the boss was convinced that this was the best solution and all joints did come out very shiney, in the end.
it really surprised me to see these handled like dishes in a sink
there was even a dish rack to hold the board vertically while they air dried.
all of us were teenagers (ie, low cost)
I wonder how reliable those displays were. but the memory of that first soldering job I had will never leave me. scrubbing boards like dishes, under hot running water - that really surprised me. it was all low density chips and TH parts and I suppose that once the boards were fully dry, all was well.
the lesson I learned was: acid flux can be used IF you don't let it sit very long and you take great pains to remove it after the process. but again, this was the late 70's..