Verified answer

MS Paint doesn't have layers, which means that once you do something on top of something else, it forgets what's under it.

You mean the pencil marks from the actual drawing you scanned in? I would try going over the lines in MS Paint, then going back with the eraser tool and erasing the smudge marks, etc, that you don't want, while avoiding erasing the new lines. Even if you do erase a little bit of the lines you do want, you can always go back over them. This is, however, very time-consuming... This is the only solution I can come up with. MS Paint isn't exactly the most convenient tool for stuff like this. ^^;; Hope that helps... Maybe try a different program, such as Paint.NET? It's very similar to MS Paint, and free to use, but you can create a layer on top of the scanned drawing so you can fix the drawing on the new layer, and can later delete the layer with the scanned drawing.

1. Sketch with a red pencil.

2. Trace with a black pencil.

3. Erase the red lines.

Select eraser, choose red, drag right mouse button over red lines.