You don’t need to render out all the values when drawing your painting. Try to limit your drawing to just the landmarks that you’ll need to aid in your painting. Look for big shapes and clearly defined shadows. The sole purpose of this stage is to aid your final painting, rather than ending up with a fully rendered drawing.
Once your drawing is finished, either erase the grid lines and then spray it with workable fixative, or ink the lines with permanent pigment-based ink. Don’t use dye-based inks like a Sharpie or a ballpoint pen, because they can bleed through the paint. India ink works well, as do Micron Pigma pens. Bouguereau would ink his drawings with a brush.