It’s a mechanical joint so acid flux is ok just wash it clean after your done. Copper is a poor choice for wrapping piano wire it’s the weakest link in the joint. Instead I use 16, 17, 18 or 19 gauge tie wire or mechanics wire in either black steel for stove pipe repair it also comes in galvanized. When you wrap it tight it will hold everything in place and allow you to make final adjustments before soldering. You need a soldering iron with a large tip so it holds the heat when in use soldering guns and small irons will melt the solder but might not tin all the surfaces. Sand everything to be soldered until bright that includes the wrapping wire before you wrap. If done right the solder will wick down thru the whole joint and remain shiny after solidifying. These are pictures of a 20 year old Lazy Ace done this way. Painted or unpainted as long as you clean the joint well there is no corrosion. Never had any of them let go either and this is used as a glider tug and on floats so it has had some pretty rough usage. These heavy duty soldering irons work better than an open flame I found mine for $29 its the large heating element that allows you to solder large joints http://www.amazon.com/Weller-SP175-1.../dp/B00002N7S4 I also swear by Staybright low temp silver solder. Most hobby shops sell it in this size.