Resistance soldering: solder w/ flux or separately?
Background first, questions later:

For those who are not familiar with resistance soldering, it is a lot like 'arc welding' where a current is passed between two electrodes with resulting heat generated in a very small area at a fast rate so as not to heat the surrounding areas. Voltage is only around 6 volts, but current is very high.
Either using a 'tweezers' type of hand held unit or a ground clip and a 'probe' type of hand unit. Depending on what is being soldered would determine the type of hand unit. For our application a 'tweezers' type of hand unit should be ideal.

You can make one of these yourselves (if you are so inclined since the power unit is nothing but a transformer);
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Or purchase one from these two manufactures (both US made); American Beauty and P-L-B.
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This site is basically a 'ad' for AB, but is helpful;
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This is about the cheapest source for the AB 100 watt basic model;
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Of course it's hard to justify the cost to solder a few 'power feeders' unless you are lucky to find a unit used as I did.
I was going to go with the smaller 100 watt unit, but I was concerned that it would limit the usefulness down the road for other applications. The 100 watt is fine for just N scale and fine work, but I now find using their 250 watt step up version I'm running the adjustable 'pot' at half current, which means it would have to be at full if it was a 100 watt station. I talked with someone from AB's tech support and he admitted many buy the 100w version only to find it wasn't enough. (this wasn't a sales pitch either)

Now the question. Since the 'key' is unrestricted conduction between the two tips, anything the hinders this causes a problem. No electrical path, no heat.
The problem is flux buildup on the tips of the 'tweezer' unit using flux core solder requiring constant cleaning of the tips.
Would it be better to use pure solder and apply flux to the work first?
I am a big fan of resistance soldering and use flux on almost all my solder joints. I have never experienced any problems with with it even applied in generous amounts as seen here where I am soldering a boiler.    
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But, that is substantially larger/heaver work then what I'm talking about by 10 fold. This is for soldering feeders to N scale track.
Elsewhere it was suggested it isn't the flux, it's the oxidation of the tip from the heat.

BTW, the link for the do it yourself is bad. Remove the space between "resistance" and "soldering" and it should work.

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