In the early eighties a UK flying school I frequented operated a number of Cessna 150, 152 and 172 aircraft.
The owner of the school took advice then from his engineers and they sprayed the inside of the fuselages, wings and tailplanes with an oil based substance similar to ACF50.
They never had corrosion problems and some of those aircraft are still flying today still based at their coastal aerodrome, the moral is, have a proper internal inspection, check the maintenance records of the aircraft you will find some good ones.