Rust under the paint

[Luke68]

As I was preparing my tail feathers for fabric covering I noticed some light rust come through the paint, as I sanded the paint away the rust looked almost like worm trails. There were six or seven places where this occurred and it was not a big deal to remedy I just wanted other builders to keep an eye out for similar problems.
On a side note my kit spent the first six years of its life only feet from the Pacific Ocean so I’m blaming that as the primary cause

[S Lathrop]

That is spooky. My bet is that what you are seeing is the result of not preparing the part properly before it was painted. It could just be a place where someone handled the part prior to being painted. In which case what you have is a local issue.

What you have here is why I am reluctant to powder coat parts. While the finish is really good, looks great and protects very well. once that surface is compromised, you get what you see on your part.

[Mark Goldberg]

We do not powder coat anything at the kit factory. For the reason that an expert on painting planes and paint in general discouraged us from doing any powder coating on our fuselages or parts. His reasons were very convincing although I am not really interested in debating all that here. And the truth is that some of the painting on early fuselages was not as good as it is now. I think now we are doing a very good job with the painting. Mark

[Luke68]

My kit was built in 2006 and like I said it spent a long time being exposed to salt air before I purchased it, all it would take is a small nick in the paint to allow contamination to get in.

[Glenn Patterson]

The pattern looks like water/oil out of an air hose & blowing it up close to chase it away to dry the surface. If you blow water up close it goes squirrely like that. If the person though that it was just water and it was clean then they may not have used any degreaser prior to painting. The result would be that the paint would not bond in that same squirrely pattern. The water just needs to get in a microscopic hole and then it would migrate along the lines where the primer was compromised. The slightest oil will interfere with paint & primer. First rule of painting is cleanliness and degreasing before each of the painting steps.

[Battson]

That looks like electrolytic corrosion. Salt plus metal equals corrosion. Even paint can't save you from that entirely, not an ordinary paint job at least.
I operate near the sea often, and I could post dozens of photos of the same thing on aluminium, steel, etc.