I have not had a problem as I do not spray a heavy coat of varnish. A box is helpful so I may rotate the sheet with figures and keep the wind at bay.
does it dull out the shine of the armor?
Not really, but then the application is more a mist. The figures are rotated a quater turn which is enough varnish. Certain colours (light) will seem bleached, but I have no problem with metal paint (Vallejo).
Follow the link and you will find a picture of the box. Since the photo, the box has been replace once.
OK I will do a test spray on some lesser noble. Was not sure if there was another product what worked to protect the paint while keeping the shine of armor. I don't like shiny figures so Dullcote was always my go to spray, I just didn't have to deal with armor. Thanks for the response.