5. Sweat, deodorant and collar marks: acid, then alkali, never chlorine
The yellow crust in the armpits of white shirts is not sweat alone. It is a reaction between the aluminium salts in antiperspirant, body oils and the fabric, which is why it builds up slowly and sits stiff to the touch.
Treat in two stages. First an acid to dissolve the mineral component: soak the affected area in a solution of one part white vinegar to four parts warm water for thirty minutes, or rub in lemon juice and leave it in sunlight for an hour. You will often feel the stiffness soften.
Then an alkali and enzymes for the oils and protein. Make a paste of bicarbonate of soda and a little water, work it into the area with a brush, leave an hour, then wash at 40°C with biological detergent. For heavy build-up, add an oxygen bleach soak overnight between the two stages.
The important rule: never use chlorine bleach on these marks. Chlorine reacts with the proteins in sweat residue and turns the area a deeper, more stubborn yellow. Nearly every shirt with a bright orange underarm has met household bleach at some point.
Grey or greasy collar marks are body oil plus product, and they come out with dish soap worked in before the wash, exactly like step three.
To stop the problem returning, let antiperspirant dry fully before dressing, use the smallest amount that works, and wash white shirts after every wear rather than letting residue accumulate over three or four.