Filling the seams at butt joins for a perfect paint finish (how do i?)
You might want to pre-treat the gap you are filling with shellac or paint so that the raw/cut particle-board is not "sucking up" the filler.

On the filler front, for painted boxes, you might visit the auto-body supply and pick up some bondo. Possibly just the 1-part "Glazing and spot putty" will do, but the 2-part (and stinky) standard auto body filler will fill larger gaps more effectively.