[Apologies in advance - this post got out of control...!]
I've not used an AB, only brush, and have managed some pretty decent finishes. I'm no expert but I've been happy with my results and had a lot of fun doing it.
This is how I'd do an early Spit... I saw you had one, me too! The classic first model (or in my case, first return-to-modelling model!).
I start with a primer coat - currently using Halfords auto primer (link), nowt wrong with it and very affordable. I'd suggest getting a grey primer unless you've got some light or bright colours - wish I'd used white primer on the Red Arrows Gnat, but live 'n learn.
Then the main body colours - starting with the lightest. So, for example, with a Dark Earth / Dark Green topside and Sky bottom (e.g. early Spit), I'd start with the Sky underside (I use Humbrol paints as they're what I can get locally and they've been fine so far - Sky is Hu23, the green is Hu29 and earth is Hu30). I wouldn't bother masking at this point. I'd thin the paints with water (you can use acrylic thinners too - just another expensive liquid I haven't got round to buying yet!), I generally go about 2 parts paint to 1 part water. I paint it on with a fairly large brush if there's no detail work; seems to help avoid some of the bristle marks. You can generally let the paint dry for a few hours - second coat can go on when 'touch' dry, doesn't need to be fully cured. Make sure you paint past the line where the topside colours will go so there are no gaps! Once you've got two coats on, have a look and see if the coverage is good enough. A further thin coat might be needed - with the Red Arrows Gnat I ended up with four or five coats I recall.
Next you need to mask up the straight lines (like along the fuselage). Bit of a pain - the paint needs to be properly cured for this, so you must leave at least 24 hours. One thing you might want to do in that time is have a stab at masking up your canopies - I see you've avoided this so far (so did I on my first models), but it makes a huge difference. Others have spoken about masking canopies recently on the forums, basically it's not as tough as it looks; a single engine fighter doesn't have that much glass to worry about. A sharp blade is the key!
OK, Sky colour cured. Mask up along the fuselage - the most popular I think is the
Tamiya 6mm masking tape, it's not as sticky as decorators' tape and far less likely to take any of the bottom layer with it when you remove it.. Honestly on my first planes I didn't bother masking along the wing edges (even the leading edges) and I was fine... I would def do it now though. Getting more anal as I do more modelling...! Then select the lighter of the two topside shades (probably the earth - but it's a toss-up here), and again do a couple of thinned layers all over the top. If you've masked off the canopy, fit this in place first so the shading is consistent with the body.
For the second colour (first must be at least touch dry, you might want to wait longer to be sure), you can either do it freehand or with more masking. Masking round shapes is fiddly, but possible... I've done both; if you're using multiple thin coats, masking is advised as it's hard to get the lines exactly the same freehand and you risk having messy edges. Best to get rid of the masking tape as soon as the paint is touch dry, less chance of paint chipping or peeling away then if you wait for ages. Having said that I've done both and not had issues.
Again, once done, wait until the paint is properly dry - 24 hours at least. Tip - keep drying planes etc under cover so they don't get dust etc on them while drying. Once you're happy, you need to seal with a varnish - I've been well pleased with
Humbrol Clear, there are threads about that around here too, as there are loads of other options. Humbrol have a good
showing how to brush Clear onto a model - do as they say: Basically, large brush, fast even strokes. Again wait until cured, then you can do the
decals: with the layer of gloss varnish, they should go on pretty easily. I do also use Microsol (
link to John's store) to really get the
decals set down right into the lines and over rivets (on older models) etc, works fantastically.
Completely optional: You might want to mess around with some weathering at this point (actually I do another coat of Clear first to seal the
decals down properly) - easiest option is to make a 'wash' of a bit of brown/rust/oilstain oil paint (just from local craft store - you can also buy specialist washes) with some turps or whatever - get it to the consistency of 'dirty turps' rather than thin paint. Sloosh it all over (alternatively you can be a bit more genteel and do a 'pin wash', just applying it carefully into panel lines etc), and once it's mostly dry, then try to clean as much of it off as you can with a few cotton buds (or a lot of them!). Should leave hardly any on the model. Amazing what a difference this makes - really ups the realism. It's just... dirt, you know! (Obviously less likely to be found on, e.g. a Red Arrows display jet than a tired three-sorties-a-day Battle of Britain Spitfire...!) You can experiment with some other effects, too - some very thin dark grey/brown for some exhaust streaks, whatever takes your fancy. Google some photos of in-action planes to see what sort of wear-and-tear is realistic. I guess you could spend ages on this; I'm happy just to do enough to suggest some hard living without troubling the 'rivet-counters' too much.
Once I'm happy with all of that, another layer of Clear just for luck. By now, the model might be looking very glossy - the way to up the gloss is simply more layers of Clear, but I prefer matt... So once everything's totally dry, I finish with a wee blast from a can of Humbrol matt spray varnish. Honestly, I don't work for them or anything, it's just what's available at the local shop!!!
Now only one job left - with a toothpick (invaluable things), prise away the masking tape from the canopy - the 'glass' bit will be clear (any leaks will easily come away with the same toothpick which is also soft enough not to scratch the plastic) but the metal frames etc will be an exact match for the body work.
I'm not the world's best modeller or anything, but I was really pleased with how easy it was to get from 'beginner' to 'early intermediate' (!!) with my painting. One day I'll try an AB, right now brushes are fine, I'm having loads of fun and that's what it's about. To see what sort of results I get, check out my Handley Page Hampden which seemed to go well, a few of my earlier efforts (all single seat WWII fighters) and the Red Arrows Gnat I mentioned.
Enjoy!