i have some from the 60's that were passed down to me. I sanded them and away i went with them. No issues even without a finish on them. I think the wood that the older tip-up were made from is treated.
I did apply a coat of lacquer finish on one of them just to give it a shine. I work with poly sometime, and i think it may be too thick of a finish, and could cause you some issues with the tip-up having a hard time opening and closing if they are put together tightly. Note, i took my apart so that i could lacquer the joints too. I used Red and green Kool-aid to stain the wood before adding the finish. It looks cool, but they lasted since the 60's without needing the finish.
