I was wondering about airbrushing a mailbox, it will probably be using a combination of Tamiya and Testors enamel paints on it. If the whole mailbox is covered with several clear gloss coats after it has been airbrushed, will that be durable enough to prevent the paint from coming off during snow/ rain ?

Yes, however, if you want a more durable, longer lasting coat, I would go with Rustoleum spray for metal or Krylon PlastiKote spray for plastics. You should be able to find both at your local home improvement store or Wal-Mart type store.

As Tim "The Tool Man" Allen would say...

https://www.youtube.com/watch?v=YQwYNca4iog#t=44

I will be covering it with a coat of rustoleum gloss.

I would recommend using a good primer. If metal, self-etching primer. If plastic, Varspray plastic primer should work.

The only problem you might run into is fading. Most "outdoor" use paint is formulated to be fade resistant. That might not hold true for model paints.