Oh I learned from a master, this guy could run his hand over something you worked on for hours and feel a low spot you could not see or feel period. He would wet it and run a squeegee over it and then tell you to lay your head right on the line of the body and move the light just so and yup, a tiny low spot. To him Bondo was something you used to make a mold for a fiberglass part you had to make for a Corvette and then it was thrown out! Metal was worked out until it needed a skim coat glaze for sanding scratches. And even they had better be at a minimum. Yea, he was tough but I am glad it was him that taught me. Of coarse that is all moot point with todays plastic cars.