I want to buy a car two or three years old. If it has seen one winter will the car still rust throughout its lifetime? I was told it would
Yes, any vehicle made of metal will develop rust during it lifetime if used outside. This is not just restricted to vehicles used during the winter. There are multiple reasons that rust will take a hold of a vehicle and these tend to correspond to different places where it will start.
Leaking rubber seals allowing water to enter the cabin or trunk area will cause damp to collect in the carpets and sound proofing which will pool in areas of the floor pan and spare wheel well. This will gradually start the process of corrosion from inside to out.
Chips, scratches and scrapes in the paintwork damages the protective glossy top layers of the paint allowing moisture to get to the porous paint layers and metal below. Even the most careful driving will lead to an amount of "gravel rash" along the leading edge of the bonnet and front panels of the vehicle.
Normal vehicle use generates any number of recurrent vibrations and shocks which are transmitted around the bodyshell of the vehicle. This can lead to rust proofed seams to flex and begin to weaken the rust protection. This eventually partially fails and water thrown up from the road during driving in the rain will get into these areas and begin to cause corrosion.
Blocked water drains can cause water to puddle within enclosed spaces in the vehicle and cause both corrosion and electrical problems. Typical areas for this are blocked sunroof drains and blocked drains within the enclosed scuttle area of the car. Typically dead leaves and other debris big enough to become trapped but not small enough to escape blocks rubber drain grommets intended to allow rainwater to run out of box-sections down to the road. This can cause quite serious corrosion if it takes hold in an area such as the bulkhead.
So, what can we do to stop this? Luckily there are several simple steps we can take to stem the inevitable corrosion. Something as simple as regularly cleaning the body of a vehicle and applying a layer of polish/wax helps massively. During the wash process, hose off or jet wash the areas under the vehicle to remove any dirt or mud that has accumulated there.
Find out where all of your vehicles rain holes are and make a point to check that they aren't blocked. This can be as simple as vacuuming the scuttle panel and inserting something into the rubber drain bungs to ensure they aren't holding water.
Regularly removing the car mats and feeling the carpet to ensure it's dry will point to any leaking door seals. Don't forget to check under the carpet in the trunk. Also look for moisture on windows that have been opened when they are first closed as this may indicate a blocked drain or a failed weather strip.
Buying a colour matched "chip kit" and using this to fill in any chips which appear on the vehicle helps to maintain the integrity of the paintwork. Pay particular attention of the leading edges of painted panels which are in the air stream when the vehicle is driven.
There are many waxy / protective products that can be professionally applied to the underside of a vehicle on a regular (every 2 years or so) which are designed to prevent under body corrosion.
Of course the best guarantee against corrosion is to keep the vehicle in a dry, warm and dehumidified environment. This could be as simple as parking the vehicle in a garage. There are extreme cases (such as the way my Mk1 Golf doesn't come out of the garage for around four months of the year) but for a car you actually intend to drive, this is probably impractical.
One thing you may want to consider with a new or nearly new vehicle is that modern cars typically come with a long (7 to 11 year) anti-corrosion warranty from the manufacturer. Be sure to speak to the vendor about the detail of the terms of this as this is probably the best peace of mind with regards to corrosion on your prospective purchase.