While I doubt that the varnish will block much UV light when the coating layer is thin it might help to reduce the gas fading (oxidising) of the OBAs. The paper you mention is full of OBA. It might be a better idea to start with a paper that has a moderate amount of them, say the Canon Heavyweight Glossy or Satin Photographic that shows less fade issues and put just one layer of varnish on top. It will not reduce the OBA effect you obviously want and might be enough to reduce gas fading of OBA and inks.
Yes, this all is not so well tested as needed. Most varnish tests have been done on Fiber/Baryta papers and not on RC papers.
I discovered something else when I sprayed two varnish types on an polyethylene foil so I could compare their skin forming (transparency, hardness etc) when the dried layers were pulled from the PE. Both acrylic varnishes but one with a white spirit solvent that evaporated slowly. The white spirit most likely was absorbed by the PE as it expanded and the surface was no longer flat. I would expect similar behaviour of the PE barriers in RC paper. The other varnish dried quicker and had not that effect. The white spirit one has been good on matte art papers in my experience as it does not affect the image. Horses for courses.
An anti-static brush remains a good tool in this business. Keeping humidity high in the spraying area helps too.
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Met vriendelijke groet, Ernst
340+ paper white spectral plots:
http://www.pigment-print.com/spectralplots/spectrumviz_1.htmupdate july 2012: Moab changes, paper sorting by name