I've been spraying the underarms with vinegar and it seems to help. These white dress shirts I'm working on are not new and the stains have been set by putting them in the dryer, but I can see a difference (or I have a good imagination!!).
I recently heard crushing 5 asprin, mixing with water and applying the mixture to the area will do wonders for perspiration stains, though I have yet to try it. Couldn't hurt :)
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