The 201 has a wet wing, meaning the normal ribs and skin have been sealed up to contain the fuel without using a separate fuel tank. Wet wings are common, but the sealant material used in many 201s at the factory did not hold up in every case. There have been some changes in the chemical makeup of avgas over the decades, and that may be partly the reason the sealant broke down and fuel started to seep out around rivets and skin laps. In any case, the only cure is for a mechanic to access the tank, scrape out the original sealant and apply new material. It is a slow, tedious and costly process, and the bad news is that it doesn’t always work the first time. However, certain shops became very good at it, and most of the airplanes that had leaky tanks have been fixed.