Nancy Reyner demonstrates mixing an acrylic glaze, using glazing medium
Nancy Reyner, the author of Acrylic Revolution, gives a good demonstration of how to mix and apply an acrylic glaze in the online video Glazing Techniques with Acrylic Paint.
Here, she demonstrates mixing the glaze. It's important not to mix the glazing medium into the paint, because you won't get the right thinness of glaze. She makes a mixture of medium, and then paint, separate from the original spots of medium and glaze, so as not to contaminate them.
Nancy Reyner demonstrates applying an acrylic glaze, testing its degree of thinness
Then she demonstrates the thinness of the glaze, and how to apply it using a soft brush. This is something you definitely don't want to use a bristle brush for! The softer brush hairs will allow a softer application of the glaze – one that doesn't show brush marks.
Nancy Reyner demonstrates how to glaze a painting
Finally, she applies the glaze to her painting. It's important not to keep working over a glaze once it begins to dry and get tacky – that would ruin the work you've already done. To speed up the process, she uses a hair dryer (a heat gun would work, too) to dry the glaze, so that she can continue applying another layer of glaze.
In her blog post Transparent Layers – Glazing vs. Washes, Nancy Reyner adds the following tips:
1. She likes to apply a little glaze in a small area with a brush, and then spread the color thinly and evenly with a rag.
2. Applying a thin layer of Golden's Acrylic Glazing Liquid, which is more fluid than the regular glazing medium, before glazing helps smooth the glazing process. While the Acrylic Glazing Liquid is still wet, she then works colored glazes into it.
3. She also suggests using Golden's Open Acrylics for mixing glazes, as they allow more time for drying. It makes applying the glazes much easier – but remember to allow each layer of glaze to fully dry before you come back with another layer over the top.