Rust bubbles
 

I was wondering if the rust bubbles that are common on older
911's are from the inside out or are they usually just from a
chip in the paint and they start rusting under the paint?

I have seen these bubbles under paint but the paint had no chips in it.
So I don't know.
It seems like if its rusting from the inside out, the hole car could be covered in rust but you wouldn't know except where its come through.

Any thoughts? thanks Mojo

Sounds like some body work was done w/o fully removing the rust (i.e. sandblasting), so it's creapt back from beneath. Quite common with lower quality body work.

When I first got my 69S...it was a very faded orange.
I wanted guards red so I started skinning down the paint....got to the primer (red oxide) then found grey primer underneath.
The problem was, the grey had the measles (rust spots) all over the car!
Apparently the grey primer was allowed to get wet...and then they just re-primered with red and captured the rust.
If I had not taken the car right down to bare metal I would have lost the car.
Be very careful about rust spots, assume the worst, look further until you can bet your life you got it all.
Bob

Well it seemed like the rust I saw was under the original paint.
So in 1976 when they started zincing porsche's, did they zinc the
whole panel underside too, or just zinc the outside before paint?

They were dipped.

Rust can come from both sides and from under or from a stone chip on the surface. Any way you slice it, Bob is right, you gotta get it out completely.