honestly once you understand the science behind it, its just like anything else. Practice makes perfect and you gain skills the more times you fuck something up, learn from it and dont do it again.
I have painted flexable parts not knowing about flex additives and had it crack apart. Plastic pieces had paint flake off becasue I didnt unse an adheasion promoter, orange peeled a job I spent hours on blocking it smoother than a babys ass, only to sand it all back off to start again becasue of lack of surface prep, done many nice repairs and great paint work on top only to have it rust or crack off becasue I left a tiny bit of rust behind the work, or allowed water to get behind it becasue I left a pin hole in my welds.
I learned from all of those.
Now I am very confidant in my work, and would have no pee-pee shakes while I take a DA sander to the side of a brand new car to shave it up or something.
There are alot of things to know about each and every step. lots of ticks of the trade to know that will save you time and make you money, and turn a shade tree repair into a professional result.
I am no pro body man, but I have alot of jobs under my belt… If I had a spot to work in I would be doing it alot more for side work. I spent alot of time in my friends fathers body shop eating bondo dust and draging a air hose around a car to pick up alot of information that allowed his family owned bussiness run successfully for over 40 years.