You have to remember "Filling" scratches requires sanding off the top. There is NOTHING (generally speaking) that is going to "fill" scratches when you sand over them making it unable to see them without first sanding it flat.
BELIEVE ME that polyester primer WILL fill 36 grit scratches with two or three coats EASY! I have done it, I have did tests where it was abused! I did some dents on an friends car with his permission as a test. I sanded filler with nice new sharp 36 grit sandpaper and feathered the paint around it with 120. I applied three coats (as I remember it was 3, it may have been 4) and blocked it with 180, then 320, then 400 then 600 and painted black basecoat clear coat over that. Six months later I saw some of the 400 grit scratches that I missed and the bs/cc shrunk up a bit. But the 36 grit, NOTHING.