Paint Measure
If you only need a tiny bit of paint, counting drops works pretty well. But, here is something I came up with to measure slightly larger amounts of paint and thinner.
A small hypodermic syringe, a standard glass eye dropper and a short piece of aquarium tubing with an outside diameter of 1/4 inch. Not all aquarium tubing is this size, and it must fit snuggly in the eye dropper and the syringe. The syringe is marked at one cm intervals, and each interval corresponds to 7 drops. When the plunger reaches the highest mark, the eye dropper will be almost full. In use, I never exceed this mark, and the syringe and tubing are never touched by paint or thinner. I have found that the rubber seal in the syringe swells when hit by lacquer thinner and becomes wedged in place. To clean the dropper, I replace the syringe with a standard rubber bulb.
This may seem somewhat anal, but I hate to waste paint, and I have become pretty good at estimating the amount that I will need for each session. I mix paint and thinner right in the airbrush cup and never pour thinned paint back in the bottle. I've had paint turn to jelly after doing this. To mix, I use the dropper that had paint in it to suck and squirt several times into the cup. This not only does a good job, but it also gets a little more paint out of the dropper and it makes it a little easier to clean.
Before spraying paint, I backflush by covering the nozzle of the airbrush with a wad of paper towel, pressing the trigger for air and GENTLY pulling back on the trigger to make bubbles in the cup. This ensures that the paint and thinner are well mixed. If you try this with a single action brush, be sure to screw the needle all the way closed before pressing for air, or you may create a paint geyser. Back the needle off slowly to get bubbles.
Decanting Spray Paint
I don't like using spray paint directly from the can because it goes all over the place and is difficult to control. I guess the airbrush has spoiled me. Anyway, I do occasionally decant it and use it in my 155 and I've never had any problems with it.
It's already pretty thin and can be sprayed without further thinning. I clean the brush with regular lacquer thinner. I won't try to explain how to decant it, but instead direct you to Brett Green's video, which does an excellent job. Be sure to look at part 2 also.
Postscript
I've never had a problem mixing paint and thinner in the cup of my Anthem. But, when I tried doing that in the Patriot cup with tiny amounts of paint, I had trouble getting a uniform mix. Whatever I put in first would run down into the nozzle and not mix well. Even with backflushing, the first shots would come out too thick or too thin. It doesn't matter much if I'm doing general coverage, but it does if I want to do detail close up work. So, when I need just a little paint in a gravity fed cup, I have begun mixing in a separate container first. Here's what I've come up with.
From left to right, they are a metal thimble, a plastic cap from a spray bottle, and a small Testors paint bottle. The thimble is good for tiny amounts of paint, the plastic cap for slightly larger amounts (lacquer thinner doesn't hurt it), and the Testors bottle for bigger amounts. The thimble and cup are easy to clean, and the bottle has the advantage of being sealable.