I have a theory! I'm going to put it to the test when I have a spare hour or so. My theory is, that when we select a colour to start a new paint job, we shake the hell out of the bottle, and then make sure there's no gunk residing in the bottom, then shake it again. This process will coat the inside of the dropper nozzle with both paint, and some of the not-quite dissolved residue. Unless the bottle is brand new, this residue will have been lurking there, exposed to what little air there is in the bottle, since we last used it, which will, in most cases be weeks! Now, when we come to squeeze the drops of paint into our
airbrush, whats the first stuff to reach the bottom of our paint cup? Yup, the gunk and residue, some of which will almost certainly be part cured.
I just cleaned a nozzle out, from a varnish bottle, (retrieved from the bin, after the last frustrations) and sure enough, there was evidence of thicker varnish, and even some tiny specks of solid material (when I say tiny, I mean tiny, gnat's knacker sized, but plenty enough to trouble a 0.3 nozzle). So, my suggested regime, for those having issues is to:
1, Before shaking the bottle, open it carefully, remove the nozzle, and, with your thumb over the opening, shake the hell out of it.
2. Wash the nozzle thoroughly, really thoroughly and push a wet cocktail stick into the hole.
3. Wipe out the washed nozzle with "Plenty" Kitchen Towel (important that it is this brand, it doesn't leave traces of paper in its path like nearly every other brand).
4. replace nozzle and proceed as normal!
Having done this, I sprayed an old model all over with said varnish, and had no spluttering and no blockage. I did use the 0.4 nozzle/needle combo, but it worked fine, which, for me using Vallejo varnish, was a first.
What a shag, but it makes sense. I will be doing more with other colours...