I purchased a new Knaus Sun TI Motorhome in November 2013 from Lowdhams in Nottingham. In November 2015, following a Habitation Service, I discovered what appeared to be small rust bumps around the edges of some of the external doors - Habitation door, Gas Compartment door, Door to Waste Water / Electrics and another small door to right of Habitation door. I wasn't 100% sure it was rust as I thought aluminium didn't rust. However, I contacted Lowdhams in Huddersfield and they agreed to replace the affected doors under warranty.
Now, I have discovered a similar problem to another door (small door to Garage Compartment - on passenger side). I again contacted Lowdhams but they wouldn't agree to do anything as they said I was now out of warranty (even though this is clearly a long standing problem which began when the van was still under warranty).
Has anyone else experienced a similar problem? What has caused it? And is it something to be concerned about?

contact knaus customer services and explain the situation to them pointing out the previous problems and that in your opinion there was a fault in the doors at the build of the van