I had a flybridge rail stanchion bolt hole let go during our record wet winter months and leak down through the overhead and down the wall.
Yes, it could be old varnish alright, the way it reacted to being damp. It really didn't damage the wood at all, just picked the finish up and discolored it. Looked very ugly but I was pleasantly surprised how easily it cleaned up with 220 grit paper.
I have another bulkhead that's sun faded. I might do it at the same time if this turns out well. Whatever finish they used didn't penetrate into the wood very deep. I'm not sure what product was applied at the factory.
Yes, I use Orange Glo too.