hi sceadwain.
thanks for the feedback. I agree use with caution!.
Thinking about the times I have had to resort to this type of flux.
I've had to use it mostly on the tags of mains/power transformers, were the manufacturers have used 'solder thru' enamelled copper wires.
Other times are when wires have been 'soldered' to brass/bronze electrical components, or when a component has corroded
away a piece of pcb track and you trying solder in a new component.
The original solder joints have failed due to corrosion due to the harsh on site environments.
As you already know, the pieces of kit that we produce don't always sit on a desk in an air conditioned office.
If the OP has a Weller iron [temp controlled??], I would suggest he removes the solder bit and looks for a 'number' [ single digit] stamped on the blunt end of the bit.
If it has a number, the number relates to operating temp control of the tip. If its a '6' for example it could be to 'cool' for the solder he is using, I use a '7', it gives that little extra heat to quickly heat the joint.
Personally I would never use plain/bare copper wire on a pcb, it tarnishes
and its a pain to tin and solder, also it corrodes in a damp environment especially if its got a few volts on it. [cathodic/anodic corrosion]. It can be varnished to prevent corrosion, but its not worth the hassle.
Regards
Eric