For this step by step, I will be using the PanOceania Teutonic Knight with Spitfire.
Primer - I prime models using a spray can. Cheap auto primer - the kind that does NOT say "filler" anywhere on it, for obvious reasons! Rustoleum Sandable Auto Primer Flat Black is my current favorite. If the primer misses any spots, I will go in and touch it up with a brush and Vallejo Model Colour Flat Black.
Base Coat - Using an airbrush, I apply a nice, even coat of Vallejo Model Colour Dark Prussian Blue. It covers black very well in a single coat, and provides a nice dark blue base to work from.
I often get asked what airbrush I am using. For base-coating and terrain, I use an inexpensive "beater" airbrush that takes a lot of abuse and keeps on working, despite my neglect: The Master G23.
I use a mixture of Isopropyl Rubbing Alcohol and Distilled water to thin the paints for airbrushing - about 10% alcohol to 90% water. Each paint thins differently, but it is generally about a 50/50 mix of paint and thinner for me (for VMC paints). I want a nice balance between good, opaque coverage, but not being too thick to clog up the airbrush. It's the consistency of milk.
Main Colour - The main color for these models is a mix of 2 parts Dark Prussian Blue and 1 part VMC Blue Green.
I find it extremely useful to mix whole bottles of these paints so that the entire army will match no matter when I get around to painting more of it.
For highlighting, I move to an airbrush that has a little more finesse - the Badger Sotar 20/20. Basically, It has a tighter spray pattern (finer needle) and a much smoother action. Amazon will have it for just $80 a few times a year, so keep an eye out.
Highlight 1 - For the first highlight, I add more Blue Green and a little White to the previous mixture.
Each highlight should cover less and less surface area, concentrating the lighter colors toward the "sunward" edge. Using an airbrush, you can use an increasingly acute angle of spray with each highlight to help inform you where the highlights would be; where the sunlight would hit.
I use the SparMax AC27 Airstream compressor. It's quiet enough that I can run it while my wife is asleep in the next room without disturbing her, and provides plenty of power. It also takes up very little space. I have a sheet of foam under it that keeps it from sliding or "walking" across the hardwood of my studio floor.
Highlight 2 - I add more White to the previous mixture for another highlight.
The dropper bottles I use are "Boston Rounds," like THESE.
Highlight 3 - This highlight is mostly white, with just a bit of the Dark Prussian Blue and Blue Green mix added to it.
And... I forgot to photograph the back for this step.
Final Highlight - Pure white is used for the final highlight.
This would normally be Vallejo Model Colour or Game Colour, or even P3 Morrow White... But pure white was sold out in those brands EVERYWHERE I looked! I picked up Army Painter white, GW White and this Reaper White. The Army Painter white was gritty, thick, with a sand-like pigment texture. AWFUL. The GW (layer, not base) had random chunks of pigment clumping up, and was too thin otheriwse (and the base is an over-thick mess of a pile of garbage paint). The Reaper is OK, but I just don't like the formulation of their paints; I have much better luck with Vallejo and P3.
It goes without saying that lumpy or gritty paint is NOT desirable for airbrushing. It will clog you up and frustrate you immediately.
Reclaim - By this point, I felt like I had lost too much of my "main" color, so I went back in with it to bring everything back to center.
This is the last airbrush stage, so I will clean that out and get ready for brush painting.
Since I had the airbrush out, and it goes so quickly - I figured I might as well use all the paint I thinned down, so I did all these steps on a batch of 20 models all at once. It only took a couple hours, and now they're all ready to be finished off one by one, or in smaller groups.
Lining - I apply a dark line in every panel, crease, crevasse or crack to define each and every shape using Reaper Masters Series Blue Liner.
The "Liners" are specifically formulated for this task, and are perfect for it. I prefer using a dark version of the color I am painting over using black, because black can be too harsh and make your models look like the Sunday Morning funnies. Since Reaper has cut their selection down to 3 colors for their Liners - Blue, Grey, and Brown - I have found that adding Ink to Paint makes a fair approximation, and gets the job done. So instead of thinning the paint with water, thin it with the appropriately colored ink to keep the pigmentation high will increasing its fluidity.
A very good-quality brush is essential here. You need a brush that holds enough paint to remain fluid, but comes to an incredibly sharp, fine point. Kolinsky Sable is the best material for brushes for this task. It has excellent spring, forms a very sharp tip, and is soft to avoid brush strokes. I use Windor & Newton Series 7 Round, Size 0.
Edging - Using the highlight colors, I add a very fine, sharp highlight along the edges of each armor section and panel.
Again, a very good-quality brush is essential to make these very sharp, fine lines.
I used these three colors for the edging - the darker areas get the darker of the 3 colors to edge them, while the brightest areas are edged in pure white.
You will notice that when airbrushing the highlights, they do not airbrush with the "full intensity" of the undiluted paint itself, so the airbrushed white highlight is not pure white, and can be further brush-highlighted using pure white.
These last two steps really define the miniature and make the details "pop."
Finally, I use VMC Black to block out all the areas that are not blue, just to clean it up so that we can focus on just the armor.