Let me throw my take on pre shading. There was a discussion on this method on another thread and some of the responses given was that real planes don't have pre shade lines painted and therefore they are unrealistic. Of course real planes don't have pre shade lines painted on, that is not the idea or the answer. The idea behind the pre shading is to fool the human eye into seeing those subtle differences on the surface of the model, and to break up monotone colors to add interest. Pre shading, IMO has to be very subtle and to just give a hint of texture and color varience. This can also be accomplished by doing post shading, which is spraying a lighter version of the base coat on the panel centers.
I do not pre shade all models so it's a matter of taste, and time spent. I like to shoot gloss black because it tends to flow better than flat colors, diluted 40-50%. I use enamels so I set my Badger Renegade that has a xtra fine tip to shoot at about 5-10 psi. Once I have the job completed, I spray very thin coats of what ever color the scheme takes, over the entire plane but making certain that I do not completely cover the pre shading. Then I go back to each individul panel and begin to lay a heavier coat from the center out and making sure that the preshade is visible as a subtle shadow.
Here is a 1/72 scale F-4J with raised lines which will benefit from the pre shading.
Notice how the pre shading helps to break up the monotone grey and white scheme and adds some interest and texture to the scheme.
SEA schemes are hard to pre shade cause the top coat colors are dark and can easily overtake the pre shade effect so it just takes a bit more AB time to get it right. I see pre shading as the DOT filter method, where the oil paint is wiped off about 98% or so, just leaving a hint.
Toshi this is a little more complex painting method but with some practise anyone can do it.