Very worth it Brian. That is a beautiful subassembly. The exhausts look quite steam punk There isn't one bit of that which isn't first rate in execution and the figures look good as well. My tip for figures is: The smaller they are less becomes more. Using a wash on the face will add creases and eye shading that we would add with highlights and shading on larger figures, but still looks as effective.
Certainly figures at this scale are difficult Ian, I'll give your idea of a wash a try, up to now I've been dry brushing highlights, which isn't really effective this small.
A question regarding your acrylic paints: Are the fairly opaque?
I've tried some acrylics through my airbrush several times and found that some of the colours tended to 'multiply' with each layer – resulting in a much darker colour than wanted. do brass and fibreglass need any special preparation for the vallejo paints.
And – do you clean the airbrush with the thinners or just with water?
They are very opaque Steve, when brushed they cover in one coat much better than the alternative Humbrol enamels which seem to have lost pigment over the last few years. When airbrushed usually one sometimes two applications is more than enough with most of the colour range, but yellows are difficult, white is easy. For anything a primer is a must to get them to adhere and stay stuck! The tan you see here is their 'Desert Sand' primer, it comes in various colours, black, grey, white and several colours. It is a water based polyurethane. They suggest 24 hour curing period to allow proper adhesion to metals and plastics - I have in the past cut corners and tried after 4 hours but it peels off, so 24 hours it is. All I do for preparation is what should normally be done, scuff the surface with a 1000 grit wet/dry paper than wipe over, I use methylated spirit so it evaporates quickly.
As I mentioned the one drawback is they are water resistant when dry, not waterproof. So when the job is finished I spray their polyurthane satin clear over to seal the paint, leave that for 24 hours and then use the only Humbrol product in my collection now - Satin Cote clear enamel so that it is completely waterproof. You could use Tamiya or Testors or one of the other enamels as a clear coat, its just that Humbrol is the only enamel available to me.
AS A FOOTNOTE.... Vallejo do several paint ranges, ModelAir which is formulated for direct airbrushing, I can't get on with this at all. ModelColor which is intended for brushing, but with dilution can be airbrushed, this is my go to paint but you have to experiment with dilution depending on how hot it is where you reside! Then ModelPremium, this is their top of the line paint intended for airbrushing only, it is said to have superior pigments giving great colour coverage, I've yet to try this one so can't comment on it.