I've just been agonising over something similar with the Airfix B5N2 'Kate', using acrylics. I've tried loads of things - different paint mixes, retarders, airbrush pressures for freehand airbrushing, various types of masking raised above the surface to get the soft edge, brush painting on top of a wet surface, etc., etc. I eventually cracked it (I think) by going for freehand airbrushing with very, very thin paint - more like thinners with a touch of paint - pressure around 10psi, and the airbrush backstop screwed down so it only allows a tiny amount of paint through. This seems to avoid spattering and pooling, and allows me to build up the colour gradually. I also remove the crown from the airbrush to get close enough. I know some folks on here can get very fine lines without doing all this, but this seems to work for me. It looks OK on a plastic bottle, anyway - I've not done the real thing yet. I'll be practising a lot before a commit to that!
