Hi Brian: I'm new to this Forum , but here goes: you can use several light sprays of workable fixatif -ie Krylon-over the watercolouring . You'll find that other materials such as pencils can then be employed over the watercolour.
However, the techniques I worked out for myself in the mid 80's worked best if the final spray was laid on after all colouring was done. That meant toning the FB print in sepia ( sometimes selectively) , carefully adding watercolour to selected areas or details by either brush or airbrush, and then completing the entire artwork with Marshall's oil colours. After drying, a final light varnish spray cohered the piece.
BTW: I can't believe the cost of Marshall's products now! I used to use the High-Intensity Set of 16 tubes, which are now only available singly and at 10 bucks a pop! $160.00 ! ..and I still have a small cardboard Marshall's box from my original set with a sticker label on it for $17.95 !! The tube size hasn't changed!