Hey Hoosier, this be off topic a bit, but it's still DIY ....
You mentioned shellac and reminded me of a "chair" that I have to cover as protection against the elements and possible cracking of the wood. It's not a regular chair, just a cut from a tree trunk that I am using as a seat. Is it a good idea to use shellac or should I go with polyurethane ..... if you happen to know ...
Actually with bark and live wood (wood between the core and bark), I would go with a UV protecting clear deck sealer for seasoned wood. Clean, spray deck sealer, let dry, and it will be good for at least a year in the worst conditions, longer if it isn't constantly exposed to the weather and sun.
If it is still green, unseasoned, some paraffin wax melted into the ends will allow it to season through the bark side slowly so it doesn't crack. Cheap candle wax and a heat gun will do it. It takes about 5 years to season this way, but should minimize cracking. You wouldn't want to seal the sides until it is seasoned.
You'd have to go with a button shellac, to get good protection with shellac, for something like that and it is tough to work with and not something you'd find unless you have some good shellac connections since the buttons are still hand made, plus it tends toward less transparent colors so it would hide much of the character of such a seat. (Button shellac makes a killer floor finish though because it is tougher than urethane and takes wax better than anything else so you can get a show car like shine that is durable.) Any shellac out of a can is a pot-shot as to if it is within its usable lifespan so I'd classify a can of shellac on a store shelf as junk, which is a shame because shellac is a great finish and easy to work with if, and only if, it is fresh.