[Cleaning up black and white scans]

Some of the scans of my drawings are on vellum which leaves a very messy background. I need to use the Erase White Paper filter to export a transparent png. What would be the best way to clean up the background? 

And then does the adjustment layer need to be merged before exporting? 

 

OK, I just tried Curves, Levels, Flatten, Erase White Paper works well. So is it so that I need to flatten first?

[Line art transparency for icon]

I scanned a line drawing as a png and place it or paste on a second layer above a transparent background. Now how do I get the white to disappear. In Photoshop I just set the layer to darken and it would save for web as a transparent png.