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! :(
Just a note: if you're using fiber paper and marshall's oils, you really don't need any kind of spray or varnish after you've finished the piece. Behind glass, any marks or 'floating' will disappear. I just don't like to put anything on my print that isn't absolutely essential.
FWIW, you don't need marshall's oils if you don't want to pay the price. You can use regular artists oils -- just have to be a bit more careful with them and thin them down so they're not so opaque.
I have stopped trying to spray a any kind of protectant onto my finished prints as a solution. I don't frame all the prints I colour which is why I tried to protect them but I use the the water colours on RC paper and oils on FB paper and that seems to work out ok.
I have also used office highlighters on RC prints for a pretty wild look as well. Definately not for every print!