From SweatSolutions.org:
“People assume that sweat causes underarm stains,” says Thomas, “But you wouldn’t have stains if it weren’t for your antiperspirant. Antiperspirants are very acidic and it’s that acidity that causes the colors on your clothes to shift.” Antiperspirant products that come in roll-on or gel forms, adds Thomas, tend to cause more severe staining. Gels and roll-ons are more watery and therefore get onto clothing more easily, leading to more color changes in the fabric.
The best way to prevent antiperspirants from transferring onto clothes and from potential staining, according to Thomas, is to apply just a thin layer of antiperspirant to your underarms and to allow it to dry thoroughly before donning clothing. It’s especially important to allow roll-on and gel formulas time to dry before dressing. Although many of us are tempted to spread our antiperspirants on thickly (thinking that more equals better and, in this case, drier), Thomas advises against that: “A stick shouldn’t work much better if the coverage is thick, it’ll just lead to more product getting on your clothes. Complete, uniform coverage of the underarm is what’s important. The active ingredient in antiperspirants works by forming very shallow plugs in the sweat ducts, just below the skin surface. These plugs form a barrier so sweat cannot come out of that duct.” The plugs, by the way, are so shallow that they come out with everyday exfoliation, such as shaving.
