What he said, but start over.
But I would not want to coat it during the stripping. To fair out the surfaces you will be removing all of the coating to get it smooth.

Can you keep it from being rained on?
I have had extended periods of high humidity, but never had that much shrinkage.
Do you think the wood was dry to start with?
All of the WRC I get is some degree of wet, I cut the strips and let them sit for a month or so (not closely stacked) to dry out.
68" is really big.