No matter how the bit is heated, it must be kept clean. The dirty-looking, oxide-covered specimen won’t transfer heat properly and brings contamination to the joint. File it or wire brush it, then keep it wholesome during work by wiping it on a damp cloth. Any bit must be tinned before use: fluxed, covered with a thin coat of fresh solder and refluxed to leave it shining silver (next photo).
Photo Courtesy Fox Chapel Publishing