Pre-shading and panel line accentuation are different things. Pre-shading is about replicating the play of light over a large surface onto the much smaller surface of a model. Look at any 1:1 scale aircraft and you won't see a solid block of colour but a subtle range of shades of the base colour. To make a model look realistic you need a way to replicate that and this is where pre-shading and post-shading come in. It is a convenient method to roughly use the panels on an aircraft for this purpose but it should not follow the panel loines too closely and it certainly must not be too neat. It is also best achieved by
airbrush. It works best in my experience by combining pre and post shading. You can also use this to replicate the bleaching effect of the sun by mixing in white with the base colour for post shading.
Panel line accentuation is a different matter and is about showing the dirt that inevitably gets into the small gaps between panel lines. This can be done by sludge washes, pin washes or as some do, using pigments. Personally I think a pin wash works best over a glossed surface which once the first matt coat is laid down an additional use of washes to create smearing of dirt in the direction of airflow.
You can see this in the picture below of my Spitty build. I did some additional oil leak smearing after the final matt coat to get the sheen that oil streaking would leave. The latter was from AKI Oil and kerosene leak weathering washes from their Air Series.
I hope that helps.
i
