Are you sketching out on canvas in pencil?? If so, stop. That's where you're going wrong. Pencil tends to show through when you paint over... paint doesn't cover carbon very effectively.
Try sketching out in either a conte crayon, or a small brush and thinned oil paint - you'll be painting over the top of it anyway. If you use a thinned out burnt sienna, you'll get a nice glow coming through when you paint over. If you use the conte crayon, a proper putty rubber will work effectively to remove any mistakes - it has the consistency of blu-tac, and will pick up bits/dust from the conte crayon. If using thinned oil paint, then a turps dipped rag can be used to remove any lines.
Neither media will give you the accuracy of a pencil, however, accuracy is not really required: instead try doing a 'working drawing' on paper first (with pencil) and then copy the drawing onto the canvas (using conte crayon or paint).