|The mottling in the first and second images is from the matte varnish that is sprayed on.|
Adjusting the blacks helps remove most of that and careful dust removal using the cloning tool can take out the rest.
When I scan I use an Epson Perfection 600 scanner which will do prints and also negatives and transparencies. I always hit the reset button to take off any scanner adjustments to sharpness, color, or levels and do those myself later using a photo editing program (Photoshop Elements 11 which came with the scanner).
When I open the raw scanned image in Photoshop I first adjust the levels with the original artwork right beside me to compare. I try to get the blacks as dark as I can without losing any of the lighter values in the image. I do the same to the white levels - get them white without losing any of the subtle grey values near them. If it is a black and white image I also strip out the colors using the desaturate feature.
Then I start going over the blacks especially doing dust removal with the cloning tool. It takes a fair amount of time to do this right. I zoom in quite a bit on the image to be sure to get it all. I get a bit obsessive in this stage to get as much as I can. At this stage I could also remove any smudges in the dark areas and also transfer lines that still may be there.
Once I have the image the way I want it I may sharpen it a bit - but not too much or the subtle areas will be lost. I also add a layer and tint it to 5-7% transparency with an olive color to bring back the warmer tones in of the scratchboard.
If I want to print the image I usually have to run a few test prints and make a few more adjustments to get the print matching the artwork - again with the original artwork right beside me.
That is my process at this time. It works pretty well for me so far. I got a lot of tips from the wetcanvas scratchboard forum, and especially Cathy Sheeter who is one of the moderators for the forum.