Brother MFC-J3540DW Online User's Manual page 482

If you feel dirt or debris, clean the glass again. You may need to repeat the cleaning process three or four
times. To test, make a copy after each cleaning attempt.
When there is a spot of dirt or correction fluid on the glass strip, the printed sample shows a vertical line.
After the glass strip is cleaned, the vertical line is gone.
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• Clean Your Brother Machine
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• Telephone and Fax Problems
• Other Problems
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