Photoshop :: Grayscale Doesn't Remove Col-erase Lines From Sketches

I can't seem to remove col-erase lines from my sketches in Photoshop.I have Photoshop CS5 and my OS is Windows 7
 
1) Scan drawing and open in Photoshop

2) Go to channels and remove the other colours than the colour of the pencil (ex. remove green and blue if the colour is red)

3) Change the mode to Grayscale (and back to RGB/CMYK if you want to colour it)
 
My problem is.. that it only goes halfway. It looks fine until step three. When I change it to Grayscale, the col-erase lines come back, just in B/W and I need to erase them anyway.

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