the idea behind finishing interior woodwork is that it seals the wood &
slows or limits the seasonal changes in it's moisture content- which is why
a perfect joint today shows up tomorrow as a crack-

Strongly suggest that you seal all 4 sides.... & nail it to some substantive
back blocking with a pneumatic pin nailer [which will leave the smallest hole]

that said my all time favorite blend in putty is Durham's which when made with
the coffee from last night and a little bit of the kids paints is tintable in a
rainbow of colors.

DD