After airbrushing the basecoat, I found a few specks and tiny (cat) hairs in the paint, which I carefully sanded out with 1200 grit sandpaper. Then using the same paints I used for his basecoat, I mixed up fresh paint and blended the colors in.
Then, I sealed Rebel in Krylon matte finish to seal in the paint before adding pastels. There are mixed reviews on using Krlyon sealer, but I've only had a couple bad cans over the years. I shake it for several minutes before using it on the model (and hold my breath that I don't have any problems). I spray outside (or in the garage if it's NaMoPaiMo and it's snowing outside), then let the model dry in the basement, which is not as cold as the garage, but still far enough from people that we're not breathing the fumes.
For the first layer of pastels, I mixed some earth pigment light yellow ochre and Pearl Ex sparkle gold. I applied this with a soft Pastel Smoothie all over. Then I mixed up another batch of the same colors and added in earth pigment dark yellow ochre. This I applied over the first layer of pastels where I wanted it darker. I then sealed with Krylon matte and let dry. I generally don't use color formulas. I usually figure it out as I go, but maybe if I did a better job of keeping track of which colors I used I might be able to fine-tune the color formulas I like. I have struggled to find light pastels/pigments for buckskins and palominos that I really like. I think I'm liking these.
Next, I used Pan Pastel 820.3 neutral gray shade for the skin tones with a small paintbrush and micro brushes. I checked all over and lifted stray gray pastel off with a kneaded eraser (I do this whenever I add darker pastel, and almost always miss some, grrr!) Then I used my golden color mixes, as well as Pan Pastel white to highlight, deepen and blend the head and body color. Sealed with Krylon matte and let dry. It is easiest to with pastels to build up color from light to dark, but the white Pan Pastel is actually decent at lightening an area, which is great for blending around the muzzle and eyes.
Layer 3 I started with a Sennelier pastel in a burnt sienna shade, not sure which one as the label is gone. I used this on his legs. I checked all over for errant color, then I did a little work on his muzzle and added more of the golden shades to try to get rid of some grain that was showing up in the dark areas. Sealed and dried.
There are a wide variety of pastel sticks out there to choose from. I've used Derwent, which are pretty hard and don't scrape easily. Sennelier and other soft pastels are very nice. You can scrape them to mix colors, or rub the pigment directly on your brush and then directly onto the horse. Generally, the higher quality (more expensive pastels) will give you a better result as the pigment content is better.
Layer 4 was more of the same, to build up color and eliminate grain, and adding black to his legs, muzzle and eyes. Things are really starting to come together, though he's ending up a little less "buttermilk" buckskin that I planned.
Layer 5, 6, 7 just adding black to his legs and using some of the golden mixes to try to blend it in.