There are two types of pre-shading that I normally use these days. Basically one can choose to highlight the edges, or highlight the interior panels. The reason this works is that blood red is semi translucent, and when airbrushed on, some of the base coat will show through. There is also
For my Ultramarines I may use the inverse method from this one, though I will need to find a good grey primer. For the thunder-chicken here, I first primed the model black, and used fortress white around the edges of the panels. My goal was to make the lines kind of wide, so my highlight color never touches black.
Painting continues, so I'll keep you posted.