An experiment: Pre-shading Dark Angels

So, after looking around the net at various airbrushing techniques, I stumbled across the idea of pre-shading. The concept is to paint your shadows (panel lines, recesses, etc.) onto a vehicle in a darker shade of the undercoat before mist-coating in your chosen colour. Assuming you apply a number of thin layers of base-coat, your pre-shading should just be visible as a darker shade of your base-coat, essentially shading your vehicle very rapidly and effectively.

Great concept, but how the heck do I do it with Dark Angels Green? I need something that's going to match the way I paint my troops, else the vehicles just won't look contiguous with the rest of the army. My normal colour process for the armour on Dark Angel troops is as follows:
1) Black undercoat
2) 1:1 mix of Black and Dark Angels Green
3) pure Dark Angels Green, leaving the previous colour in the recesses
4) highlight with Snot Green (blend, edge highlights, etc. - depends on the detail-level I want)

So, given I start with a black undercoat, it's kinda difficult to paint a darker colour to pre-shade on a vehicle...What happens if I use a medium grey undercoat? That solves the pre-shade problem, but will it result in a brighter Dark Angels Green than my normal troops? Is there an alternative that will work instead?

Time for an experiment :)

I've used an old Rhino as a test vehicle, primarily because it has non-Chapter doors glued on and I've decided all of my vehicles will have DA doors from FW. I intend to compare the following:
1) A pre-shade using medium grey as the undercoat colour
2) Use my troops process on the vehicle to compare results

Without further ado, here's where I got to in a few hours of messing around last night. All paint is Vallejo Game Color and thinned with Windex. Numbers in brackets are droplet count for mixing.

1) Base-coat Cold Grey:Windex (60:20) at 20psi

2) Pre-shade Black:Windex (60:20) at 5psi

I noted here that pressure was too high at 10psi so I lowered it to 5, but still found it too high (it was blowing wet paint around). Not sure if this is a pressure issue or the paint was too thin, will need to research.

3) Light coat of Dark Green:Windex (60:15) at 15psi

I noted here the paint was waaaaay too thin!

4) Light coat of Dark Green:Windex (60:5) at 15psi

I noted here it was a little better, maybe even a bit too thick, but I didn't wait long enough between coats (I'm impatient!)

This is where I left it so it had a chance to dry completely. It probably needs two more really light coats, probably thinned somewhere between the two previous tests. Will post updates as I progress.

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In the other corner, the progress is a lot harder to see, and not as far down the path. I managed to get one coat of the darker green colour over a black undercoat. The colour I mixed was absolutely perfect to match the first stage I use on my troops, so I'm pretty happy, but it needs a second coat of this mix before I proceed to DA Green.

You can't really see the colour from the photo, but here it is anyway :P

All in all, a good night of experimentation, certainly learned a little about my airbrush. You can see from the last photo I was also messing around with thin lines - I am far from perfecting this, but I at least have a starting point of knowledge to build on now (5psi, about half an inch or so from the object renders a line about double the thickness of a felt tip pen, which is good for a start).