everytime ive used the gas method, i havent scrubbed it afrter dowsing it. dowse it in gas, rub it in with the rag, let it evaporate and you should be good.
White gas and then some soap+water later has done the trick for me about a half-dozen times (to the skin, if it got on my clothes, I get a fresh change), but I know I only have a mild reaction to the stuff anyways.
With MW it sounds like he's blistering and boiling, something that I don't think poison ivy/oak has ever made me do, so he might have a reaction just looking at the stuff because he's so sensitive.