Common problem my neighborers octavia rusted after 2 months but he managed to get the dealer to respray them.
Its hard to say what to use on them as they are constantly get hot then cooling causing the paint to fail, mine are still rust free but will go the same way in the end.
Hammerite will work but getting the rightcolour is the problem and it could make them look as bad as having rust,it may be that you paint them with a good coat of the right colour paint and do it again when it reappears,