"Washing powder" is not a term that's used here. Can you be more specific? Is it the kind of chlorinated, semi-abrasive stuff you use to scrub stains off of sinks? Is it what you use to wash clothes? I've tried the detergent I use for hand washing dishes. I've tried the detergent I use for washing floors. I've tried the stuff I use in the dishwasher. I tried the detergent I use for washing clothes. None of them would remove the residue left after cleaning with isopropyl alcohol and salt. Acetone wouldn't do it, either. I'm tempted to try hydrochloric acid, which I have for cleaning brickwork, but I prefer not to work with it. I thought of putting the bong in the oven for a high-temp cleaning cycle, but I'm afraid that might just bake the residue in.