Ya might try this....using a 9' roller drill a really small hole in the end piece on the frame side (round plastic piece that actually holds the cover in place) find a finish nail that will fit in that hole snuggly use a foam cover and roll the varnish out,put the nail in the hole and that will stop the roller from spinning effectively turning it into a 9" back brush. 'Brush' the varnish out to a glass like finish.
I would also reccomend thinning with a little penetrol to delay setup time.
Learned this when I worked at a sign shop back in the day. It's the smoothest way to finish something other than spraying. Only problem is it's impractical except on big flat surfaces like 4X8 sheets of plywood or table tops.
Good luck wid it