I have used Golden Archival Varnish with UVLS with great success. It is specifically made for watercolor and other mediums too.
Normally, watercolors should not be varnished at all. You're altering the character of the surface when you do so. Is there a specific reason you want to varnish it?
I was told to use a workable fixative on watercolours if you need to, however if you intend to frame it behind glass then it is usually preferable to not use any fixative as it can alter the colors slightly. But its probably a personal choice thing, so test it out first on a scrap piece of paper just to make sure it won't ruin your gorgeous creation. I use crystal kote workable fixative which has a uv coating on my colored pencils and pastels but I haven't tried it on my watercolours yet, but it does say on the spray can that it works for that, I would suggest that you just use sparingly and test it first. All the best
I agree, personal choice; but just be aware that if you plan to enter watercolors in juried shows, many of them will not accept w/c with varnish ... in some technical sense or other they are no longer "pure" watercolor if you've varnished them. If that's not a concern, however, go for it.
thanks so much for so many great suggestions :-) good to know about the juried shows and that possibility! this one in particular is just a personal issue that Im having with shading a multi media piece of work. I'm shading with graphite which as we all know smears quite a bit with water. Ive used it both ways...shading with the watercolor over the graphite to use it as part of the shading and then also being precise about where I put the watercolor so that I didn't run into the graphite...but it doesn't look right when I try to shade after....... wondered if I could use the fixative over the blending stump shading and go from there. guess the best is to just give it a go! Ive already blown 3 paintings in the process that I would've loved having as finished pieces. Its a quest now ;-)
I use a cheap hair spray from Tesco - it works fine. Brian