from my experience with cable its has been beneficial to unwind the 6 or 8 main strands and place in the fire and let it sit at a dull to mid red for a few minutes to burn out and paint or the thick grease type stuff i have came accross. then wire brush the crap out of it when it cools to get the scale off along with traces of anything else then put the wire back together and do as you did heat to red, brush and flux, then get to welding and set the welds. i have had pieces that no matter how clean i thought it was there was still something keeping it from welding. hope this helps.