there are some sprays for watercolors and/or for oils and acrylics, may be best to look at acrylic sealer, yet know that this may alter colours down a tone , and suggest doing “test on patch work first”, look for spray sealer, brush on may smear image, and fresh circulating air , or one coat of spray to hold, and then quick brush over, don’t go over twice right away or curdling may occur.?

water colour doesn’t bond to itself well like oil and acrylic, thus cracking, that usually why they use on paper, yet if on canvas, so be , try to find fine woven canvas, then suggest sealing back , if cloth, if on board, then assumption is that gesso has been put on already, so from there, you want a water soluble paint that has paper texture, maybe even less gesso on canvas, allowing paint to soak in a little, like drywall primer,
maybe something like this by gluing tissue paper on surface,

other option is only to leave as is and put thin matted glass or plastic over top
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