I just bought a digital soldering station because I am about to replace a bunch of connectors on batteries. There is a rheostat that allows temperature control. The tip is only 1/8" in diameter and has a sharp tapered point. The heat is constant once set so no need to hold anything over a candle, then hurry to get it under the sagging wire.
I tried it on a parallel pair of heat stretched sprue wires, both with a lot of slack and curved in opposite directions. Started at 200 deg and worked up slowly. 360 deg seemed to work well, not too quickly, but the fine point and constant heat allowed me to straighten out both wires very nicely. No way I could have done parallel wires at the same time with a heated wire or blade.
I bought the iron for the RC side of my hobby, but had I seen it work, I would have bought it just for tightening rigging. Bought it with extra points, a solder pump, tip cleaner and shipping for 66 bucks. Made by BAKU and I got mine from Motion RC. Highly recommended for any rigging method or material that required heat to tighten the wires.
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