"Can I use a fixative that I bought to use on my sketches, when I want to draw my picture onto the canvas before I paint with acrylics?" -- Kathleen
I wouldn't use fixative because it's not designed to be part of the base layers of a painting. You could use a clear acrylic medium to seal the pencil, applying a thin layer with a broad brush, working lightly (don't scrub it on) and trying not to smear it too much. I wouldn't use spray varnish either because, if it's the removable kind and at some stage in the future the painting is cleaned (which involves removing the protective varnish on the top of the painting), it can end in disaster.
I actually wouldn't bother sealing the drawing as it adds another step in the process of creating a painting. If you like doing a detailed drawing, try doing it with a thin brush and some paint i.e. draw with a brush. This'll dry and you can overpaint it easily.
Another alternative is to use watercolor pencils for your sketch. This will "dissolve" into the paint, but watch out as some watercolor pencil colours can be quite strong and change the colour you're putting down significantly.You can also take an eraser and rub out most of your drawing before you start, leaving only the most crucial lines visible. Or do your detailed drawing/plan in a sketchbook and then only a minimal drawing on the canvas giving the main lines.
Remember that once you start applying paint the drawing is going to get less and less visible anyway, and what you've done with the paint then guides you. The initial drawing or sketch is just a starting point -- a painting isn't a colored-in drawing.