There are a LOT of ways to paint a portrait.
I use several different methods (and sometimes combine them) depending on what I think is best at the time.
Sometimes I even remember to take pictures of the process.
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This portrait of Gwyneth (20"x24", oil on linen) won the Certificate of Excellence at the Portrait Society of America's International Portrait Competition in Philadelphia this year (2008).
Here's how I did it:
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This a drawing on Acetate. I use prepared acetate instead of tracing paper because I can see through it. Believe me, it comes in handy later in the painting if my lines begin to "wander" and I need to correct.
I use a "Sharpie" Permanent Marker as it makes a clean line and doesn't smear.
I use a sheet of graphite paper and a ballpoint pen to transfer the drawing to the canvas. Acrylic paint will cover graphite (pencil). Oil paint will not cover graphite so if I were painting in oil, I'd need to erase my original lines and replace them with a Sharpie line.
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I used acrylic paint for my first layer. In order to do this, I needed to use an acrylic primed linen - acrylic paint will not stick to oil primed linen and will not be archival.
I use Golden Matte colors. I try to stick to the dull halftone, neutral earth colors.
For the color "white" I choose a dull warmish neutral - about the value of a brown paper bag. Nothing will be lighter in value than this.
If my subject has light skin, I use the same paint and color for the skintones as my "white." If my subject has darker skin, I deepen the value accordingly.
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I may need to put on more than one layer of paint in order to cover the canvas and make it flat - like a poster. It is a good way to lay down my basic composition and make a definite statement of shape.
Two thin layers are much better than one thick layer. And I try not to leave ridges.
Posterizing is a good way to see if a composition works. This layer could be in oil but acrylics are a faster way to saturate the canvas.
I chose a medium value paint for all the objects and was thinking "halftone" (the space between light and shadow). Once the white of the canvas is covered up - it doesn't look so dark.
I always establish black and white immediately in an underpainting. It will help all the other mid-range values fall into place.