The sputter I’m not sure on.  Has the environmental conditions changed there recently?  Has this 50/50 mix worked fine with this specific pot of paint before?  Every paint is different, and there can be differences between batches and pots of the same paint.  As a pot gets older it’ll often get thicker too.  You might want to experiment with the mix?

As for paint not sticking, are you washing your models properly before painting?  The only times I’ve seen a model repel paint was due to mould release agent remaining.  Or at least that’s what I assumed.

I’m no expert, so I’m expecting to be corrected on a point or two of this, but here’s my own experience:

Most whites are very ‘chalky’. They have a tendency to dry and clog faster than other paints. It doesn’t seem to matter what brand. Greys can also be like that.

If your paint is not adhering at this point in the process, it’s not the model that’s the issue. It’s either the primer or the paint you’re using. The effects around the rivets look like pooling so it’s possible that, at least for those sections, there was too much wet paint applied, or the airbrush was too close to the model. It’s going on ‘wetter’ than you want.
As for the speckled pattern, again, you’re going to get that more often with whites, I believe for the same reason they dry and clog faster. Environment and other factors can affect all of these things.

There’s a few of things you can experiment with. Try increasing the PSI, might help with the clogging, but pull the airbrush back some as well. You’re not doing detail work so no need to be careful with overspray, etc. You can get lubricant for the needle. That will help with ‘dry tip’ where the paint dries on the needle.
To cover the model in a way that avoids some of the pooling and speckling, try multiple very light coats.

All of this, of course, takes some patience.

Firstly, my advice for painting yellow.
Don’t use a black or white primer, use brown.
Yellow works really well over a brown primer, you’ll see a massive difference!

I’ve found with most airbrushing problems there are two solutions:
1) The brush needs cleaned more thoroughly
2) Adjust the mix of the paint.

1) What are you cleaning the brush with? If it’s just water you need to use some form of airbrush cleaner.
I use two different types, one is vallejo airbrush cleaner (looks a lot like water), which is my main cleaner, in addition to this I use a solvent based airbrush cleaner that comes in a spray can. This is for the more stubborn to reach areas and only gets used on “deep” cleans.
Sometimes your brush will need to be “totally” disassembled (not to the point where every O ring is removed, use common sense when taking it apart for cleaning), but certinaly more than just taking the back off and removing the needle.
paint can get in the places you least expect it. ESPECIALLY if you’ve had a blockage at some point, as the paint has to go somewhere! so it’ll usually end up somewhere it shouldn’t inside the brush.

Once you’re sure it’s 100% clean…
2) The mix of paint,
this is a really tricky one to diagnose and fix online, as there are lots and lots of things that can go wrong here.
the standard answer is to thin your paints to the consistency of semi skimmed milk,
if you do that, there’s less chance anything can go wrong, but it’s still quite vague.

Do you mix your paint in the brush? or seperate? I tend to mix mine in shot glasses, then pour it into the brush as I need it. I also don’t overfill the cup on the brush, if there is a blockage, I want the minimum in there as possible.
If you mix in the bush, there’s a good chance you’ve forced a lump of the thickest stuff way down into the brush first before your thinned stuff, giving you an inconsistent mix (overly thinned at the top, not thinned enough at the bottom), and exponentially increased your chance of a blockage right from the word go.

Is there any reason why you use the citadel stuff? I’ve not heard great things about the citadel air range, but honestly never tried it myself.
My collection of paints is a mix of old GW, vallejo & Army painter.
By far my . favourite is vallejo, I have very little issues with vallejo and the range of colours available is astounding, no matter the colour there will be a vallej equivalent!

Paint not adhering tends to be a dirty model, usually mold release agent still on the figure.
Best course of action is to use warm soapy water to wash the model before priming.

Hope this helps.
Thanks

Chris

“I generally dismantle the brush and clean it before each session. I use Acetone and a kit I got for cleaning them.”

You should be cleaning it after each session, not before.  That way nothing has time to dry inside.  It also means you’ll have minimal chance for residue from any cleaning agents still inside to ruin your paint.  It also gives it time for any moisture inside to evaporate before use so it can’t alter your mixtures.  I might be overthinking some of these points, but why take chances?

I have Iwata Eclipse (gravity feed, .35mm nozzle) and the spattering on your photo looks familiar. Most likely cause is that the nozzle is not actually clean. It might work well with paints with finer pigments, but with white the flow is not normal: the paint seeps out from the nozzle slow and is carried only with the air when outside the nozzle, almost like an external mix airbrush, resulting in a spray that not smooth as normally with Eclipse. The same could explain the painting not adhering, the thinned paint hits the model without mixing with air, thus not drying normally.

Have you cleaned the nozzle mechanically, i.e. not just with cleaning liquid? Eclipse nozzle is easy to clean with a reamer or a needle (don’t use your airbrush needle, you might easily bend it) . Make sure to get the side walls, not only the tip. Also clean the nozzle cap properly.

If you’re sure the nozzle is clean, the check the following:

– does the paint have lumps? Put some on palette, swirl around with a brush, tilt the palette. The paint should look absolutely smooth. Some people filter or sift through their paint to avoid lumps.

– is the nozzle cap properly screwed in? Don’t use force, but it should be fairly tight. If loose, it can cause the problem I described first. A

– is the nozzle cap seal (black O-ring after the threads) good?

– is the needle tip bent or the nozzle tip damaged?