The "dirty secret" of modelmakers is Future Floor Polish, which is an aqueous-based clear acrylic. I've coated both canvas and paper prints with it, using an HVLP sprayer and up to three sparse coats applied a few minutes apart. Since it dries to a textured gloss finish with this application method, it works extremely well on canvas, and remains flexible as well as very durable (can't scratch through with a careless fingernail). I haven't shipped prints with this yet, as I am still doing long term testing including sun exposure, but all results so far indicate it is viable and very inexpensive compared to other aqueous finishes. (Reports of Future going yellow, when followed up on, are invariably the substrate going yellow underneath it.)
Both canvas and paper should be sprayed after mounting. With paper prints on watercolour media, I have done experiments in wetting the paper thoroughly then mounting it as if it were canvas, on stretcher bars. As the paper dries it shrinks, it becomes tight as a drum, and rewetting it e.g. with a sealant as above makes it saggy, but it tightens up again. Still, stretched paper is about as fragile as it gets no matter how much acrylic is on it, and I wouldn't recommend this technique for anything other than self-amusement.
For high dollar prints or repro work, something like Premier's EcoShield is probably the best way to go, as it has been thoroughly tested on a wide variety of media. Very easy to apply, although quite expensive.