Underarm stains are one of the toughest stains to remove because the salts and acids from sweat combine with chemicals in the deodorant and actually change the structure of the fabric. For future reference, deodorants that don’t contain aluminium should help to prevent this from occurring, as this compound is what causes the yellow marks.
To remove sweat stains we would usually recommend the use of solutions made of baking soda and white vinegar, but you might want to try other cleaning agents such as household ammonia, which is alkaline and will help to break up the minerals. Create a solution of equal parts ammonia and water and dab it on the stain (do this in a well-ventilated area). Let it sit for an hour and wash at a cool temperature. You could even try meat tenderizer – its natural enzymes are really good at removing tough biological stains (also blood and urine), but don’t use it on natural fabrics like wool or silk. Dampen the stained area with water and apply half teaspoon of meat tenderizer using your fingers. Let it work for an hour, then wash at a cool temperature. If the shirts are coloured, test these methods first on an unseen area to prevent fading.
For white shirts, you could also try aspirin: crush and dissolve two aspirin pills into 100 ml of warm water and soak the stained area in the solution for 2 – 4 hours, then wash thoroughly. Do not use chlorine bleach, as it reacts with the protein in sweat and will actually darken the stain. Letting the garments air dry will help to bleach natural fibres like cotton and linen; but be aware that sunlight may damage polyesters.
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