The Dave Platt method that's been used successfully by many modelers is to mark your panel lines with a thin Sharpie pen, and then apply very thin "ChartPak" drafting tape (.020") to the butt-joint panel locations after your "grey coat" is down...then apply 3 to 5 coats of finish color over the taped panel lines. Remove the tape before the final coat is 100% dry...then lightly sand and clean the seam when dry. You're left with an indented panel seam that will present a nice scale appearance when you paint the entire model. The seam can be further enhanced with weathering techniques if you choose.
This is found on the "Detailing & Painting Techniques, Vol 1" tape/DVD. I strongly recommend Dave's "Black Art" series for anyone getting into scale modeling:
[link=http://www.daveplattmodels.com/]Dave Platt Models[/link]
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