Cotton is a natural fabric that many people prefer to synthetic fabrics. Since it is made of fibers, the stains can be a little more difficult to remove. Do not just throw the sweater into the laundry and expect everything to wash out. And most important, do not use hot water or the dryer while the stain is visible or there is a good chance that you will set the stain.
Treat the spot immediately after it happens or as soon as you notice it. Scrape any residual matter from the surface of the sweater to keep it from working into the fibers during the washing process, using a butter knife or spoon.
Identify the cause of the spot -- whether it contains grease, animal or vegetables dyes like coffee or wine, or protein. Treat the spot as if it contains grease if you are not sure of the cause.
Treat grease spots by dabbing a few drips of dish soap or laundry soap directly on the spot with a clean white rag, until you have given the grease a chance to emulsify. Keep changing the cleaning area on the rag to lift off as much of the grease as possible.
Rinse the spot under lukewarm water to remove the particles from the cotton sweater, running the water through the fibers from the backside to push them out instead of into the fibers.
Pour a small amount of white vinegar directly onto the stain to wet the area and loosen the stain. Let it sit for an hour before running through a cold-water wash cycle.
Inspect the cotton sweater for the stain after it has finished the wash cycle and before you place it in the dryer. Pour a few drips of hydrogen peroxide on the stain if it is still visible and watch to see if it bubbles up, showing that it is acting chemically on the stain. Let it sit for 10 to 15 minutes before rinsing with lukewarm water.
Dry the cotton sweater flat on a rack.