Re: Rust spot forming...how to treat?
Sorry I don't really have the answers but somebody should be along soon who does.
Paint codes are usually around the shutlines of the boot or tailgate on the other Alfas.
It's a bit unlucky you have a spot on a join.
Typically you would wire brush off the loose stuff, brush on rust eater, leave it 15 mins, wire brush off, rust eater/wire brush over and over until the metal's really clean, then rust converter e.g. Kurust or maybe someone can help with one that works then Zinc primer, then base coat, colour coat, laquer, job done.