A Few years ago I bought a new transporter van, It came from swansea area and was apparantly stored near a railway line, after a few months when I polished it the surface felt rough, so contacted VW and there man came along to inspect, he used a magnifier to inspect these minor pimples which under the magnifier were actually rust eating into the paint work problem was you couldn't polish it out, so in the end changed it for a new one, they said the brake dust from the train may have caused it.