I'm trying to determine how much material remains as a residual film on the inside of a 55-gallon drum after empyting it of its liquidcontent. I assume the amount that remains will vary by viscosity, temperature and time the drum was upside-down and also by composition of the drum (steel, plastic, urethane-coated etc). Does anyone have data on this (either models or empirical data) or any references to the literature? One rule of thumb I've seen is 1 to 4% of the original volume remains but I can't find anything to support this. Any help is appreciated.