Like anything else, it just takes practice! I've been working in body shops since I was 14. You'd be surprised at how much you'll pick up simply watching someone fix cars. Been full time since I graduated high school, 10 years now.
But I know you do good work so my beloved P5 will be in good hands when she's in your garage getting her rust repaired. Plus I get to help so I can learn.
Since you don't do much rust jobs, I know of a place where I had my rust repairs. Found a list where everyone we can look for and compare Ohio Auto repair shops.