Vent: the old bench guy in Wichita had me chase a bad ground for 3 hours

Last Tuesday I was working a nav radio squawk on a Cessna at our shop here in Wichita, and I had already pulled the unit twice looking for the fault. The senior tech on the bench walked over, tapped the airframe ground strap at the tail, and said the radio was fine, the return path was not. I had been trusting my meter probe on a painted surface and getting a reading that looked okay when it was not. It took a wire brush and 20 minutes to make that radio work, and now I scrape every bonding point first thing. Has anyone else been burned by a clean looking ground strap that was really just a bad connection under paint?