I might have posted this before but anyway I want to share what i do to see if a panel is flat . I just did this on my 40 ford coupe deck lid that I had previously
repaired the area where the handle area was mutilated. What I do is get the filler and primer as smooth and flat as possible
, then sand with 320 grit , followed by a 3m tricut 3000 grit pad ( spel) . The pad is usually used for their buffing system . it is
a six inch foam pad with a 3000 grit surface. I dab a drop or two of water ( or you can try it dry ) and sand away. It will make the primer almost glossy and you can check out your progress . If you still need to flatten just sand right over it add more filler or high build primer or whatever you need . To be honest , I have never used an sort of guide coat, I usually feel to see if it is strait . I have
painted for 30 plus years ( not every day) and have always had good strait
flat panels but this new way I came up with beats anything I have seen . Anyway it works for me . With the proper placement of lights you can get a hint of what the final finish will lk like .