First time scribing panel lines

I use  some different things to scribe the line.  For the clean up I take a toothpick and, at an angle, take a slice off with a sharp hobby knife.  Run the little "chisel"  just created back and forth in the grove.  Just "abrasive" enough to clean and smooth the groove.

I scribe with a heavy sewing needle, chucked in a pin vice, and use a thin (flexable) piece of metal for a straight edge. I start out by dragging the needle, slowly and softly along the straight edge, then drag it along a couple more times, applying a little more preasure each time. 5-6 "drags" usually does the trick. I then sand over the lines with 400 grit, run the needle through again, sand, needle, then sand again, using lighter grit. To get all the sanding dust out of the lines, I run a polishing disc(Dremel) over them, also polishing the plastic at the same time.

Dissolve the rough remnants of plastic on the edges of the scribed lines.

what's the purpose of running liquid cement along each line?

scribe, sand, scribe...I Run liquid cement along each line after you've sanded them down. If you think you need to scribe more to make the line deeper, don't. Its easy to end up with a trench looking line. It takes lots of practice(for me anyway), and a good too. Some are just junk. I use a Bare Metal scribe and I like it.