Re: My first ever 1/35 figure Face!
I know I am coming very late to this party, especially considering your other thread about face painting with oils, but... I will mention a couple of things.
First, the problem you describe with the thinned paint migrating to other parts of the face sounds to me like you may have too much paint on the brush. What I do, and what the "experts" recommend, is after loading the brush with paint, I remove the excess paint by placing the tip of the brush against a cloth. Capillary action takes out the excess and leaves you with a good amount of paint on the brush. I do this when highlighting and shading uniforms so I thought it worth mentioning.
For eyes, I use a modified method to what Shep describes. I place an eye shaped portion of a lightened skin tone on the eye, then go back and add the pupil with either a small brush or a cat's whisker. For cat lovers out there, I do NOT pull them out, I find them around the house where the cats sleep. When using them for painting the "root" end works well, it is very small and holds just enough paint for a 1/35 pupil. After those two steps are done, I typically have to go back and paint over portions where I placed too much paint, getting the shape of the eye the way I want it. Of course, I often miss and paint over the whole dang thing which means, back to step 1.
I do all of this with Vallejo acrylics and once dry, I paint the rest of the face with oils.
The last thing I would say about doing eyes, if you are anything like me, be ready to do them over and over and over.
I hope some of this helps.