[Karin Wells]

1. Should underpainting always be used as the basis of oil portraits?

Heavens no! As Michael explained so well, there are many ways to paint. I like to use a grisaille underpainting in a portrait for any flesh that shows, i.e., hands, feet, etc.

I presume the next step would be to apply color to the underpainting - is this done in glazes, so that the values are retained, or pretty thickly once you have established the value range?

Glazes are never applied thickly. Glazes are transparent colors and these added layers will probably darken the value of what lies underneath. Only light (opaque colors) are to be painted thickly in the top layers.

2. I noticed that Karin used a mixture of raw umber and titanium white. Are there certain underpainting colors that are particularly complementary of flesh tones? I have heard some artists mention green as a good undertone for skin.

Once again, there are many ways to achieve flesh tones and Covino's method is classic.

I personally like to begin with a raw umber/white mixture that equals the value of the flesh, i.e., in a range from caucasian to a person of color. A thin glaze of one or more of the following will oftentimes nail a skin tone: burnt umber (try first), raw sienna, and/or raw umber.

If a glaze doesn't work, wipe it off and keep experimenting. I use either Liquin or Galkyd Lite as my medium for this. After the initial skin tone is glazed to be correct, the color and values are then matched with more opaque paint and scumbled in thin layers over the areas of flesh. This can result in a "porcelain like" look.

Prior to building an area of light with thick opaque paint, I like to apply a thin glaze to the surface to "juice it up" before I begin.

3. Related to the above, is there a particular rule concerning the best color to use in an underpainting that will vivify the final coat if it is not flesh colored (e.g., a red hat )?

I am not sure that I understand this question.

Michael has done a really excellent job of explaining underpainting methods. I suggest that you experiment a little and see what works for you. There is a book out called "How to Paint Like the Old Masters" by Joseph Sheppard. So far it is the best that I have found. (However, I intend to check out Frank Covino's book as it sounds wonderful - thank you Michael.)

Chris Saper's book, "Skin Tones" is out now and I have found it to be most useful.