I use 1/32" "chart" tape, layed down where the lines should be. Then apply another couple of coats of whatever primer you are using. Sand the whole mess down to the tape with 400 grit or finer. Peel up the tape and viola! recessed panel lines that are all of uniform depth and width.

If you use the scribe method you run the risk of getting too deep or not deep enough. the tape method works the best.

Hope this helps.

Oh, by the way, the tape is used/available at your local newspaper. They use it for layouts.