My father was an SAC in RAF prior to war. He had several reels of the copper wire used, all of it was covered with dark brown shellac. Knowing how much oily sooty fumes came out of the engines, it wouldn't take long for the sticky coating to develop and hold the dirt on the wire. R/T ops used to ask for the wire to be wiped during maintenance, just to give them a sporting chance of receiving signals, remember it was all done in morse on bombers initially, spoken R/T was reserved for the fighters, being single seat in most cases.