Airbrushes Single or Double action?

Yeh.  I have been watching the sale page for the last few days to see if the one I want gets onto the sale page. :)

I picked up two Badgers through their "Garage Sale" section of their website.  I've had them for 5 years and never had a SINGLE problem with them, so don't worry about that section having junk that Badger is trying to get rid of.

That's what I was thinking.  I have shaky hands now though.  And I am just afraid I won't be able to handle the "throttle" as you put it to keep the flow even for both air and paint.  I am thinking I might grab a Badger 155 Anthem, 150, or renegade rage double action, or a 150 series single action.  A lot also depends on price.  I am watching a NOS Badger 150M kit (The Artist Gift Set).  It's unused but older and still mostly in it's plastic.  It has a few hours left on eBay so I will see what it gets too.  If not that, then I will just wait and get something on the Garage Sale on the Badger site.   I am pretty sure my Wife is getting me a compressor with a regulator and pressure gauge for my Bday on 2/2 so I am only going to worry about the airbrush itself.

Short answer?  No, the single actions are not going to give you the fine control by design.  Control being the operative word.  The double actions control both the paint flow AND the air flow.  Think of the double action trigger as a throttle.  As you pull back, you're opening up the "throttle" allowing more paint to flow through.  Of course, you can get a fine nozzle single action and get pretty good results, but then you'd have to use that fine nozzle to make multiple passes very close to cover larger areas or swap out your tip to a medium or high flow tip.  Sort of like my Paasche H single action.  I love it for covering large areas (like a 1/48 C-130 or something like that) but would be concerned about camo patterns on a 1/72 A-37 Dragonfly.  Those are probably extreme examples, but I just pulled those from looking at my stash...  At least that's my take on it.  I'm sure others with much more experience and knowledge will have better answers.