How to separate out markings written by black pen,pencil,sketch pen

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I have this image containing some text written with mix of black sketch pen, black pen and pencil and i want to eliminate black sketch markings. What would be the best way to do it? I tried thresholding and then morphological methods but didn't get required result. I am new to opencv and interested in knowing what would experts do. What is the general strategy to tackle this or similar type of problems?

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