There are a few problems here :

It's a 5 year old car, which you didn't buy new, you've only owned it for a few months.

In the prior 4.5 years, it could well have been in an accident which required new doors. If the replacement doors weren't properly painted then the problem could well have come from there.

There is no way of knowing if it has been in an accident previously. There will be no records unless the car was declared a cat C or D write-off.

Your contract is nothing - repeat, NOTHING, to do with VW/SKODA, and everything to do with the garage who sold you the car. You have owned the car for less than 6 months, and therefore the law views the rust as 'present or developing' at the time of sale. As such, the garage who sold you the car is ENTIRELY responsible for putting it right.

I'd suggest you take it back to where you bought it, and tell them to fix it. If they refuse, get the work done, give them the bill, and if they refuse to pay take them to the small claims court to get the money off them.

However, do yourself a favour. If you have to get the job done yourself, get the work done by an independent bodyshop (you say you know 2). You do have an obligation to 'mitigate costs', if it comes to a claim in court.

So the response to the OP suggesting that all four doors have been damaged seems unlikely at the least.

For one doors are often skinned and not fully repaired, so a very easy spot for any paint shop. VW/Skoda would have rejected the claim if there was any sign of repainting also, and again a very easy spot with the use of a paint depth guage.

A quick google shows up loads of simuliar issues so it looks like its a skoda issue and the reason why they are not question the cause of issue.

The paint warranty is 12 years, so it reason under the sales of goods act and warranty for the paint to last 12 years mostly rust free.

Therefore reguardless of you only recently purchasing the car. Its reasonable expectation for a proper repair to ensure the body work lasts 12 years not 8month/1yr