As long as your panel lines are deep enough, you don't have to do any scribing. I rarely scribe panels except for limited areas where they can be a little shallow, (ie. the tops of the doors). The panel lines on Sidney's Vette do look pretty shallow, so it's probably just as well that he left it alone. If you use acrylic craft paint & the results are less than desirable, it's an easy fix. A little warm water & a soft brush will clean it right out, so you can either make a second attempt, or just forget about it.
It is a pretty simple technique that will add a lot of realism to your builds.
Steve