A side note before we get to the info here though. I do not have an air brush (though I hope to get one sometime this year), I lay basecoats entirely by hand & have for years. With that in mind, this mix may not be good for an airbrush, I really have no idea! Just wanted to toss that out there before someone makes their airbrush unhappy!
On with the tutorial! Since Nocturno is going to be a rich dark bay, his base color is a mix of burnt sienna acrylic & clear gesso, thinned with water. You will want to pick or mix a base color as close to the body, or middle color, of your chosen finishwork color.
You will need:
* Acrylic in the color or colors desired
* Clear Gesso (I use Liquitex Professional)
* A sealable container
* A stir of some sort (I use a popsicle stick).
In the picture below, the first layer has been laid down on his head, neck, shoulders, & first half of his barrel. It takes many layers to completely finish a good base coat.
I start basing at the head & work my way back, usually leaving the tail for a hand hold. If your chosen model does not have a tail suitable for a handle, you have to improvise, using legs which will have to 'catch up' to the painting at every step. Since Nocturno is going to be bay & thus his legs are going to be black, I didn't bother basing them & can use them for a hand hold too.
Usually when I a basing, by the time I have gotten to the rear of the horse, the head is has dried & I can start over again. Don't get frustrated if your base looks really splotchy, you need to build up many layers to keep from getting brushstrokes, so this it totally normal for it to look horrible! Just take a look at 'Rastus' between the first few layers & the final result.
Make sure you no longer have ANY thin spots in your base; if you move onto painting while there are some weak areas in the base, I guarantee that you will see them in the finished horse, so make double sure to catch them all!
Once the base is absolutely solid you are ready to move on to actual painting!
Happy painting all!