When trying to help someone learn something I've found it's helpful to have good examples of bad examples. That's why I did this painting. There was one issue I wanted to tackle in particular, but got a second one as a bonus. Oh frabjous day!
I think I'll start with the bonus issue that sprang up. When I was in school a professor or two warned against drawing with lead pencil for underdrawings for paintings. Generally speaking they felt it was best to either block in your drawing with paint (diluted, of course) and a brush or use something like a colored pencil or pastel. The logic being you could bias the underdrawing towards warm or cool depending on your intention with the final painting. Also, whatever you put down as a drawing had that pesky tendency to mix with the paint you put on top. So, if you use a lead pencil for a drawing, well, then you're going to grey down all of the colors you put over it. And that's exactly what happened here. The painting just feels dead, for want of a better word. At least as far as the colors are concerned. Thankfully, there are ways around this.
Primarily I do my underdrawing in pencil (or graphite transfer, which amounts to the same thing) then I spray fix the drawing - and I spray fix to keep the drawing from running away when I apply a color wash later. When the drawing stops smudging I apply a coat of acrylic paint to tone the canvas and bias it towards warm or cool. I use workable fixatif for the spraying stage in the interest of speed. I've found that the acrylic wash tends to bead up and not stick to the surface if I use crystal clear for the sealing stage. I could use an oil wash on top of crystal clear, but I don't want to wait for it to dry. The acrylic wash also serves as a final sealant on the surface. If I have a false start in the painting stage I can scrub everything back down to the drawing and acrylic wash without much in the way of trouble and start over.
Now, to take a step backwards, before you seal a drawing and start painting, you've gotta do the drawing. The important thing here is to make sure you get the drawing right. I cannot stress this enough. If you aren't happy with the preliminary lay-in, don't start painting. No matter how well you render your subject over a bad drawing, it will still look wrong. I remember listening to people say they'll "just fix it when [they] go to paint", but it seemed to be more effort than it was worth. Why create extra hurdles for yourself to jump over when you're painting? As one of my professors would say, "if the frame of your house isn't stable, why would you start furnishing your bedroom?" Or something like that.
So I did a bad drawing. The anatomy is so very wrong and the composition is crap. I feel the rendering is fair (could be better, of course) but it just looks wrong. Nothing short of shifting the entire painting to one side or starting over entirely would have been able to save this. And that's the point. If it isn't right, start over. Another one of my professors used to tell us all the time, "You've got to slow down in order to go fast." I find as long as everything is in place before I sit down at the easel I can get through a painting much more quickly. Which is preferable over trying to fix everything that's wrong from the start as I go along.
That's a very long-winded way to state two simple guidlines:
1. Make sure the drawing is solid.
2. Seal the drawing so you don't contaminate your paints.