My Dad used to use the paraffin in naptha ("white gas") solution as a flotant applied on the stream. He had one of those small hourglass shaped bottles, with a little cork ring in the throat as I recall, that he filled with the mix he made up himself. Drop a no longer buoyant fly in, shake vigorously, remove and false cast it dry. It did a good job of cleaning slime off flies and seemed to work quite well. Perhaps not the most environmentally friendly technique, but those were the olden days.