I have never tried it but on an episode of "trucks" they showed using a razor blades to gently scrape the top off of a run. The theory was that when trying to sand out the runs, you actually sanded thinner areas that naturally formed right around the run from the natural pulling of the paint as the run followed gravity. By scrapping with the blade, they only removed the high area of the run itself. Take it for what it's worth but it made sense to me.
We used that trick a lot reworking the runs that came out of the main booth when I was painting Cessnas. What they didn't show you were the nicks and gouges that result from the starting & stopping at the end of each stroke of the blade. No matter how careful you are you will get them. We scraped the runs since it's faster than sanding AND safer then sanding around all of the rivets. You are correct about the paint being thinner above and below the run. That is why I told him to completely sand out the run. If you don't it will still show.
