One of my potential solutions was to finish them by hand, instead of zenithal work with the airbrush.
Here's the result of the hand painting over the airbrushed helmet:
I'm thrilled with it, honestly. There's contrast, but it's not cartoony. Details are crisper, and the white is smoother and more vibrant.
How did I get this effect?
I first applied a wash of very thin Reaper Master Series 'Ultramarine Highlight'. However, I simply couldn't get the wash to stay in the seams of the helmet. I was using water to thin, and it just wasn't working. Then I dug out my unused bottle of Winsor and Newton Flow Improver. I've had this stuff for years and never used it.
I loaded a spare brush with some of the liquid, and made a small pool of it on my palette. I then loaded my brush with the medium, and thinned a small section of the Ultramarine Highlight paint. I picked that up in the brush and used it as my wash.
It immediately clung to the seams in the helmet, as you can see above. The process was messy, so all of the other surfaces had a tint of blue as well. I gave it two or three goings-over before allowing it to dry.
I went back over the helmet with more thinned white after the wash had dried, cutting the color back to the thin lines you see above. I then hit the metallic areas using MSP Shadowed Steel, Badab Black wash, more Shadowed Steel, then Hones Steel, and finally Polished Silver.
I also reworked my eye lens colors. I used to use MSP Pine Green, Leaf Green, and Pale Green from the Warm Greens Triad. Instead, I used MSP Forest Green, Grass Green, and Jade Green from the Cool Greens Triad, plus a white dot for a lens effect. I applied the same to the targeter.
All in all, I'd say it's a definite improvement and a total success. I'm thinking that maybe all airbrush, all the time just isn't going to work for me. I think I'm going to end up doing my majority of colors using the airbrush, but going back with washes to fill details and contrast and then cutting back the washes with my main color.
I also took a few minutes to put together the Marine's eventual bolter:
It's a targeter from the IG Heavy Weapons sprue on a regular bolter. It took a lot of fiddling to put on, as the underside of that tiny targeter bit is sloped some and makes it want to point downward. It's supposed to look like an ACOG sight. I think I need to file off the iron sight at the front of the bolter, though. What do you think?
I'm also considering cutting off the sickle magazine and replacing it with a box magazine from a Black Reach Terminator's storm bolter arm. I have enough to choke a horse, so sacrificing a few for Sternguard isn't a huge deal. Good idea, or leave it as-is?