:-) Hi Kyle,
Your question is almost impossible to answer, because there are so many variables involved that what will work today, may, not work tomorrow. This is because one of the variables can be the ambient temperature. Also thinner/paint ratios depend greatly on the paint and thinner used. For example if I were using Lifecolor acrylics the ratio would be 70/30 or even 80/20, thinner (water) to paint. This is because Lifecolor paint is very thick. If I was using Tamiya acrylics (which I have never airbrushed) I would expect to use very little thinner or none at all, and thin with Isopropynol. Something else that will effect how thin the paint needs to be is how fine the line is, that you want to paint. The finer the line the smaller the outlet will have to be. Pigment plays a part here, if it isn't fine enough, it wont get through. Modern paints are very fine so your probably in luck. The smaller the outlet the more thinning you will likely have to do (again depends on the paint) the more thinning you do the less air pressure you need. Also heavily thinned paint will not cover well and would need to be built up gradually, in layers. This is a good thing, because it gives you more control. Of course all this is dependent on how fine a line you are looking for. Newtothegame is right that you need to get in close, but if the paint is to thin and/or the air pressure is to high then you will get the "millipede" affect. When I'm pre-shading I'm looking for lines about 3mm and NTTG's ratios would be about right, but adjusted to the paint I'm using. Air pressure also as NTTG suggests, but sometimes I’ve found that, oddly, a greater pressure is required (as much as twice ie 20psi [I actually work in bar, so I think I'm talking about 2 bar]) when you might think that lowering would give you better results. If I'm painting, say, a German mottle on an aircraft, (akin to painting a line .5-1mm), I would have the paint very thin and the air pressure very low (.5 bar, or less) and get in very close. My airbrush, at the moment is a single action syphon feed Badger 200. I'm looking to get a gravity feed double action. When I do I expect to have to re-learn the ratios and pressure settings that I use with my current airbrush. The only way to be able to answer your question is by using your airbrush, with the paint you use and in the conditions in which you use it. Even then there would be no hard and fast rules.
So you will need to find what works, with the paint you use. There would be many ways to do this but I would suggest starting with a very thin mix and low pressure. Gradually add more paint until the paint wont flow, then increase the pressure, gradually until it does flow. Add more paint, until it stops, and so on.
Something you must remember is that different surfaces absorb paint differently, so if you do any trials on card or paper, when you try on plastic things will be different. Use free sources of plstic such as margarine tub lids. Do clean them first though.
I hope I haven't put you off, but I didn't think that a simple "practice makes perfect" would have been of much use either. It really is about getting used to your airbrush, I'm still learning/improving after 20 odd years. Rest assured though that this whole airbrush thing is pretty easy, just have an open mind :-)
I would like to know how you get on. Do take NTTGs advice and practice before committing to a model :-)
Mal
