No, you can't do that. You will ruin the watercolour. Frame it with glass, that will protect the watercolour.

Tick

The painting is too big for a frame and glass. It is 30" high and 88" wide. (4 panels attached at the sides). I want to affix it directly on a wall, like a mural on paper, and protect the surface. I've thought of putting a large strip of plexiglass over it and screwing it into the wall. Does plexi come that large?

Thank you so much.

Good idea with the plexiglass. I think you can have that cut to size, the same as glass.

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I know you probably will not be able to buy it here, but at least you know it is sold like that.

Tick

There is no varnish that is appropriate for watercolor on paper. The Plexiglas idea is good, but if you want to mount the piece(s) directly onto the wall, I would recommend that you have them hinged onto a museum board backing to which mounting hardware can be installed. It is not advisable either for conservation or valuation purposes to pin or glue artwork directly onto a wall. As for the Plexiglas, it should be engineered in such a way as to stand proud of the surface of the work, i.e. mounted to the wall with spacers. Glass or Plexiglas should never be in direct contact with any artwork or photograph. Good luck.
Udayan