Generally speaking, it's very difficult to scribe a clean, straight line in putty. On occasion, I've been able to using the back edge of a used #11 knife blade. It took a few very light passes to be able to do it, though. However, in most cases, the odds are against you. There is a another way. If the panel lines on the kit are raised, you can replace them with stretched sprue. Once painted, you won't be able to tell them apart.