Wood is like your skin it needs to breathe. Don't try to seal up the butt end you won't be successfull and then when the stock does take on moisture from the hunt it can't escape very well so it swells and bubbles the finish.
Your wife's good hutch, buffet and dining room table are the same. The moisture content of the wood is greater in the summer and less in winter as it absorbs moisture dependant on the relative humidity in your home despite what type of finish it has. Dump a cup of coffee on the table and leave it for a while and the finish is screwed. The oil finish's are usually easier to repair.
This is why in my opinion and experience the oil finish's are best for the wood. They repel a lot of moisture but what is absorbed by the stock can slowly escape the same way, without causing damage enough to have to refinish the complete stock. The wood can breathe.
The end grain of wood always absorbs and releases moisture at a higher rate than the side grain. Hence the issues with wood stocks and the butt end. I would remove the butt plate as mentioned to let it dry. When this has happened give it several heavy applications of tung oil or what ever you choose let each soak in well then put the butt plate on a go hunting. The oil will slow down the moisture from getting in but also let it breathe.
Stock finishing has been one of my interests over the years. There are 2" thick books written on the subject. Long story short there is know known finish that I know of that will 100% protect a stock from moisture.
Thats one of the reasons they invented synthetic stocks, there impervious to almost everything including moisture. Rather ugly though.
