A couple of thing regarding extension cords/lamp cord strands:
Tarnishing/turning green will depend on the quality of the extension cord, whether the individual conductors are varnished, the PH of the water you fish and whether your skin acids are like mine and you can tarnish stainless steel.
😉
The diameter of the individual strands depends on the overall gauge of the stranded wire cord and the flexibility desired by the manufacturer.
The most common extension cords are either 16 or 18 gauge stranded wire. In an typical 18 gauge stranded wire cord this equates to anywhere from 7 to 19 individual strands of 26 AWG to 30 AWG wire.
A 16 gauge extension cord is typically anywhere from 7 to 19 strands of 24 AWG to 29 AWG wire.
As a point of reference, UTC Ultra Wire comes in 5 different gauges which measure as follow:
Large - 25 AWG (0.0179")
Medium - 27 AWG (0.0142")
Brassie - 32 AWG (0.008")
Small - 34 AWG (0.0063"
Extra Small- 38 AWG (0.004")
Ultra Wire is also varnished, spooled and comes in a many colors.
The bottom line with old cords is you don't know what size you have until you count or measure strands and you won't know if the strands are varnished until they oxidize or not. I used old cords when I started fly tying and threw away the green ribbed flies a few years later...
Folks can use and do what they want, but I never had an issue with tarnsihing or oxidation after switching to Ultra Wire, I like all the color choices and it is spooled which makes for easy storage.
When I factor in the time to strip old extension cords, count strands and figure out a way to store the strands, it isn't worth the hassle to me to save a few cents on a single color of wire.
As always, YMMV...
BTW - Another wire I use is
Beadalon Artistic Wire, a coated wire sold in craft shops. It is also available in lots of colors and spooled. I never did a price per foot comparison to Ultra Wire but they make a few colors I use (like true purple & true brass) that are unavailable from Ultra Wire.
It's good stuff and available from many sources!!