Most rust starts to form inside a panel. There are various reasons for this: dirt and moisture, and lack of factory paint treatment are the major causes. Leaving your car outdoors without shelter, in rain and dew, will hasten the process.
Any observant motorist visiting an overseas city, located in a warm, dry climate, will soon notice the absence of perforation rust in the local vehicles. When you think about it, moisture is the problem. So, how do we solve this long term?
Consider how often you have seen a rusty steel gearbox or steering knuckle. When was the last time you heard of a car engine sump rusting through? Most likely never! But these parts are all subject to moisture, salt etc. and are unpainted.
Why then, do they not rust?
The answer is of course - oil! This is the basis of rust-proofing today. Oil repels water. Whether the oil is dirty or clean, it still has the property of repelling water, and sticking to metal. All we need do to ensure that our rust does not continue is to saturate the inside of rusting panels with oil.
Don't waste money buying new oil. The oil drained from the engine or transmission of your car will work fine. The thicker the better. Most garages will fill a can for you (free) if you do not like to change your own oil.