I use Humbrol.
and I mean the act of pre-shading a model, often by spraying black (or dark) from underneath and white (or light colours) from the direction of the "light source". You then paint over that with a transparent layer of paint that colours the model but retains the shading of the lower layers.
To be fair, it can be done. If you keep the layers of paint thin enough.
I still think in terms of hairy brush, but this kind of thing is easier with an airbrush as you don't need to worry about brush-strokes.
I did use this technique (with a hairy brush) on my Wave Serpent. But the result on the blue is very subtle:
The metallic blue, gloss finish, and the interference green are all stronger effects.
Shading:
Humbrol enamels I find are quite opaque, Testors seem to be more transparent. But that's just a general thing, some specific colours have better or worse coverage. I've tried a few enamel reds and yellows and none of them cover as well as Tamiya acrylic flat red and flat yellow.
This is true. Although I have one yellow that covers quite well; Trainer yellow (24). It's not an in-your-face yellow, but is certainly yellow and serves as a good base for brighter yellows without having to do a layer or two with white mixed in.
I haven't seen any Testors in any shops I've been in, but then I generally go straight to what I know (humbrol).
I have tried Revell Enamels, but didn't like them.
I can zenithal highlight with enamels or acrylics. The only tricky thing with enamels I find is when you thin them right down for transparent airbrushing they're more prone to pooling.
True, and beyond that some colours will precipitate from solution/suspension and be useless.
Don't know if water-based paints do this...?
I find even though enamels take a long time to fully dry, they thicken up quite fast. So to do fine detail work I have to pay much more attention to thinning them frequently to keep them flowing well. Then as you say, the surface tension can act against you if you thin too much the paint will naturally seep in to the crevices instead of staying on the details.
Also the slow drying can be a pain in the arse with fine detail because you often want to swap colours, do a quick edge highlight, do a quick shade.... you just can't do that as easily with enamels.
Even on aircraft where I sometimes prefer enamels, I still swap to acrylics to do the fine cockpit details.
Yeah, getting the right consistency is important.
I use old lids for mixing paint in, instead of a dedicated pallet, and I've left some open to thicken up if I've gone too thin (I use a second spare lid, upside down on top, to keep a mixed paint good for longer).
But when too thick, a drop of white spirit and all is good.
It's mostly because it can still be manipulated long after applying it. To blend the green in to the beige on that genestealer I based the model beige, painted the green in to the crevices and then cleaned off the brush and used just the slightest hint of white spirit to smudge the green in to the beige. You just can't do that with acrylics, even if you add a ton of drying retarder to keep them wet, they will go grainy when you try and blend them like that.
Yeah, this is a technique I don't think I could give up.
The smell is a big negative for me. I also keep my thinner in a jar and just rest the lid on it, but the paint on the brush itself stinks and I often have my face very close to the model when doing finer detail stuff so I inhale more fumes than I'd like.
If you use enamel thinners rather than white spirit, it's not quite as bad. White spirit has an annoying lingering smell, if you get it on your hands (or brushes) even after washing them with soap and water the smell will still be present.
Yeah, having worked in chemistry labs, I'm quite "immune" to chemical smells.
It's certainly not for everyone.
Out here I can get Vallejo acrylics for the exact same price per bottle as Humbrol enamels, $4.50 AUD. I can actually get Humbrol acrylics even cheaper again but I hate the pots even more than I hate
GW pots

Testors enamel, which I prefer, is actually more expensive even than
GW paint locally.
Tamiya's the cheapest out here, both their enamel range and their acrylic range I can pick up for $4 a pot.
Interesting. And the moral of the story is ... Price will depend on where you are and your proximity to the company that makes/sells what you want.
I always wince when I see Australian prices for this hobby.