I have swienty national and have always varnished all contact surfaces plus applied petroleum jelly to aid separation of boxes. My oldest boxes are 17 years and still going strong.
I use exterior varnish or yacht varnish. Not sure what the difference is. I have had national poly's for last ten years and no problems with varnish. A smear of vasilene once dry makes for easy removal of frames.
Just been to Bookers (Cwmbran ) and they had a pallet of slightly damaged 25kg bags of sugar. £15 per bag. I took 75kg so still plenty left if you are in the area. With this rain yu may need it.