I think I would be using Bostik "Seal N Flex"

available in lots of colours and made for sealing vehicle bodies I believe.

frank

Or Sikaflex Marine, or Sikaflex 227


Sikaflex® 227

Sikaflex-227 is a one-component poly­­urethane sealant designed for car body construction, curing on exposure to atmospheric moisture to form a durable elastomer.
Use

Sikaflex-227 is suitable for sealing, seam sealing, simple bonding as well as for vibration reduction and sound deadening measures in crash body repair and car body construction.

Suitable substrate materials are metal primers and paint coatings (two-part systems), metals, painted plastics and plastics.

All the aforementioned sealants will attract dirt - this I know from personal experience. The best method is to use one the aforementioned sealants but keep the sealant gap to an absolute minimum.
Good Luck.

sikaflex gets my vote, best thing since cold beer.

Cheers

Ever notice that black silicon attracts light dust and white mould whereas white silicon attracts dark dust and black mould.

Being unaware of the exact problem makes it hard to recommend anything but if its only to seal rather than hold maybe some butyl mastic will do the job.

Kev