Hello Julianne:
Covino's current method is to use Retouch Varnish to both seal the underdrawing in charcoal and to bring any sunken values up in the monochrome underpainting.
I suspect that the reference to using workable fixative is old knowledge. These days he would recommend retouch varnish for that.
If you have an underpainting, then my recommendation would be to only use the retouch if you have areas that have sunk in. If your values are right, then just start painting in color.
