The panel lines on the underside are a result from two techniques, the preshading applied prior to painting the light blue, and an application of pastel dust.
Step by step.
1. Preshade over primer:
2. Light blue (and yellow of course, which is masked at this stage)
3. Pastel dust applied with a brush.
4. And finally, the excess pastel dust is worked around and removed with moistened q-tips.
As you might notice, the same was also done on the uppersides, but due to the darker colors and the unexpected need for heavier coats, the preshade effect was all but lost, leaving the most of the panel line accenting to the pastel treatment.
In the end, this is like applying a wash, but I prefer this method because it's much more forgiving, the pastel dust doesn't dry like paint washes, it can be applied dry (for exhaust & gun opening stains), or brushed on and worked with water. On the latter, you can go from just removing excess dust to using more water, making it more like a wash.
This is best applied to a flat(tish) finish, pastels don't stick to gloss surfaces. Also, apply the final flat (or whataver you like) coat carefully, preferably misting on several coats, to keep the pastels intact.