Hello I make a lot of boxes and do not have the facilities for mitered edges,
I always have a visible seam when I make a butt join. I fill it with fine crack filler and it seems to shrink into the seam as it dries, and when I sand it back it is always apparent after painting plus it roughs up the particle boards finish around it. This is "no shrink filler".
I have been looking for some kind of high build primer or something more thin to fill the scratches and seams, any hints for me?
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.
You might want to consider using some yellow wood glue in the joints as you assemble the boxes. (The glue will work better for wood or plywood than it does for particle board). The glue will help to fill any minor gap in the joints.
Another technique, after the glue is dry, is to sand the faces of the box after assembly so that the corner joints are nice and even.
