We have threads now and then about pre-finishing parts to make final finishing easier after assembly. This is a really simple example but, I can't think of the last item I made that did not involve at least some pre-finishing. Especially large china hutches or bedroom sets; I will frequently spray shellac on the interiors of the carcasses to balance moisture reactions with the outer finish throughout the year.
At any rate this is just a small lift-out tray for a box. The box didn't originally call for one but, I am on a mission to make useful items out of my scrap. I simply tape off the areas where glue will be used later and pad the finish on the areas that will be hard to reach or difficult to deal with after assembly. The green painter's tape works fine for this sort of thing. I carefully lay the tape along the glue area and then come back and burnish it with my finger nail or a stick of scrap. I remove the tape almost immediately after applying the finish.
And just so you can see where this is going . . .
. . .