Hi Toshi, another great looking build taking shape. I learned the same way you did about pre-shading. I thought skipping the set up and cleaning of the airbrush by using a pen or brush on the panel lines would be fine, until the top coats went on and then, like GM said, I could really see the demarcation between black and the top coat. Yuck! With the airbrush, you can feather the pre-shade out so it will give the subltle, blending effect.
For me, easier said than done, I am still a big novice at it and will be trying it again on my Zero.
Keep on building and posting.
Scott