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How to remove pen marks
Treat the stain as quickly as possible.
The longer you let the ink sit in the fabric, the harder it will be to remove, so take action as soon as you notice the stain. Ideally, the ink will still be wet when you treat the fabric.
Apply a pre-wash treatment solution. There are a wide variety of pre-wash treatments available in the cleaning aisle of the grocery store, where you purchase laundry detergent and dryer sheets.