To pick up light markings, use Control-Up Arrow (Windows) or Command-Up Arrow (Mac). To ignore light markings, use Control-Down Arrow (Windows) or Command-Down Arrow (Mac).
If there are too many stray marks, Control-Shift-Up Arrow (Windows) or Command-Shift-Up Arrow (Mac) to remove them. If small details aren’t showing up, use Control-Shift-Down Arrow (Windows) or Command-Shift-Down Arrow (Mac) to reveal them.
If you still can’t get a clean scan (too much “noise” in the original, for example), you might consider using a free app like Adobe Capture to import scans from a phone or tablet into Inkscape / Illustrator / Affinity Design for complete control.
Another thing to try: take a piece of paper and cut a rough hole in it that would allow your desired subject to show through but not the distracting background. Put that over the original when you do the trace.
Also, I’ve found that shiny materials don’t trace as well due to the reflection of the lights. If you can reduce the glossiness of the piece it will give you a better trace.
There’s another trick you can use which is sort of bogus but effective: select and delete. I had a trace with some areas of speckles, so I just did a rectangular selection (click and drag) to get the bits and then backspace/del and they were gone.
It’s been a little while since I’ve seen any replies on this thread so I’m going to close it. If you still need help with this please either start a new thread or email support@glowforge.com.