Hey guys this is a fantastic thread.
Thats probably the best way ever to combat pencil or graphite bleeding through and the process that I have used myself too, is to paint a thin layer of white on top of the initial drawing. Other way is to use a hard grade pencil.
If you are painting on a very dark canvas, which I have had to do in the past on some of my oceans, I swap the lead pencil for a white one which works fantastically.
Here is an example of what I did while working on my Sacred Land painting which featured my friend Wendell Deer with Horns.. It is all about Imagery and messing around with old photos and making changes..
1. First job was to take Wendells photo and then add a background, in this case the Black Mountains, a sacred place. I then added elements that I thought would further the message that the land was sacred, a buffalo, an Eagle from stock photos (paid for and free imagery).

2. Next job was to age the photo, I chose sepia and also applied a
'glow' to the overall image. I also took bits out here and there to make the imagery seem more balanced, composition wise..

3. Next job was to add a message to the top, as he wanted it to include this.. I chose my wording and put what I thought he might like to read each time he looked at it, and also to sharpen and add a little more shadow to the overly bright image.

4 . Finally I got to work on the actual painting itself in watercolours. Added the campsite and faded out the buffalo, which in the final photo manipulation had become unrecognisable. I also made Wendell look younger too, to which he responded, ALAN!! You star, I look handsome in this!! hahaha Wendell:)
Wakan Tanka guys:)

"Why do we call our planet, planet Earth, when it is almost entirely covered in water"? Arthur C. Clarke.