I agree.
After filing and fine sanding, the joint may look good. But polishing will determine the end result. Once polished, try the 'finger nail test' - gently run the edge of a fingernail across the joint, moving the entire length of the joint The finger tip and nail are very sensitive, and any irregularities should be detected. If there are any, then lightly sand again, or, if very noticeable ridges, perhaps file again, followed by fine sanding and a polish.
For polishing, I use 'T Cut', a car body colour restorer, no doubt under different brand names in various countries, but a good liquid metal polish is similar stuff. Use this on a soft cloth, not too much or it'll run all over the place, and polish vigourously, applying more polish as needed. Then keep polishing, until all residue is removed. Wipe off and polish again with a clean, soft cloth, followed by a damp cloth to remove any dried residue, followed by a final polish.
The plastic should now be shiny, and any uneveness will be visible.
If done properly, the joint should have virtually disappeared. Once primed and/or painted, no joint line should be visible. However, some shades of paint, particularly acryclics, are fairly 'thin', and might need a second or third coat.