weathering painting question (bringing out lines)

Yuppers. pre-shading is the way to go.  But in order for it to work properly, the basecoat that you spray over the pre-shading needs to be very light, so gently and lightly mist the paint over the preshading until you get just enough coverage to make it look painted, yet the pre-shading shows through.  Follow that up with a wash to accent the actual panel lines.

You can replicate the staining that is going down the side of the hull with tiny dabs of artist oils, then load up a flat brush with turpenoid and let the oil paint wash down the hull. 

Depending on the paints you're using, you may need to put a protective clear coat so whatever medium you're using for the wash won't destroy the paint job.