[HPsquared]

Given limited absorption rates, does that mean varnish etc helps keep the internal moisture content more steady over time (and therefore less variation across the wood internally as well)?

[DannyBee]

It depends on how vapor permeable they are. Some of them are good at resisting liquid water but not vapor, and some are good at resisting both.

But all things being equal, yes, they generally can only help keep moisture content more steady over time.