Soak the ink-stained sweater in a bucket of milk overnight. Ensure the ink-stained portion of the sweater is submerged in the milk to help loosen and remove the ink stain.
Remove the sweater from the milk and rinse thoroughly with cool water. Roll the sweater in an absorbent towel to remove excess moisture.
Lay the sweater on a flat work surface with the remaining ink stain facing up. Mix 2 tsp. white vinegar with 1 tbsp. cornstarch in a bowl. Spread the paste over the remaining ink stain on the sweater and leave it in place until the paste has dried. As the paste dries it absorbs the ink stain.
Brush off the dried vinegar and cornstarch paste. If any ink stain remains, blot it with a clean sponge dampened with dry cleaning solvent until the last traces of ink are gone.
Fill the bucket with cold water and stir in 2 caps full of a mild detergent made specifically for sweaters and other fine washables. Place the sweater in the detergent solution and gently agitate it by carefully dunking it up and down in the solution.
Pour out the detergent solution and fill the bucket with cool water. Soak the sweater in the water to rinse out the detergent. Gently agitate the sweater in the water. Repeat this step until the rinse water remains clean.
Dry the sweater by rolling it up in an absorbent towel to remove excess water, followed by laying it to dry on a sweater drying rack.