Rudi wrote:

Try bead blasting and then zinc chromate, works great on corroded aluminum aircraft parts. Might be the same on pot metal.

I wonder if treating it with Alodine would help stop the degradation, Then the zinc chromate? How about phosphoric acid then the zinc chromate?
 

Pot metal seems to corrode jut like aluminum, because of that I would agree with Rudi and Ron 68. You need to cover the alodine with paint or it will still corrode. Paint the whole thing inside and out.

Has anyone else had this problem? It's the first I've heard of it.

Mine are originals, and the car has been repainted, if there was an issue, it was never mentioned to me by the painter. 

Toploader wrote:

I have an ongoing issue with the paint pitting and blistering on my original headlight fender extensions. I had the car painted in 2003 and eventually they started pitting and blistering. I had them repainted by the same guy a number of years later and eventually they started pitting and blistering again.
Most recently I took the headlight fender extensions into another body shop to be stripped back to bare metal and be repainted. The body shop has just advised me that they think the casting is degrading and this is what is causing the pitting and blistering of the paint.
The body shop had them for about a week and tried three times with painting them and an epoxy primer/sealer was used.
Has anyone else had this issue? Am I faced with purchasing some reproductions?
 

which one are you needing?

Out of curiosity, where was the car built?
Where did it live most of it’s life?

BobE wrote:

Toploader - if you go the reproduction route, remember these early 64 1/2 fender extensions are different from later models ... if you're interested in keeping it original.

Wow, I never knew that. What's the difference?

Toploader wrote:

BobE wrote:

Toploader - if you go the reproduction route, remember these early 64 1/2 fender extensions are different from later models ... if you're interested in keeping it original.

Wow, I never knew that. What's the difference?

Funny story, I had read years ago, and saw pictures that were supposed to show the difference, but I really couldn't tell the difference.  It was only a couple of years ago while at a local cruise a guy pointed out that I have one 64 1/2 on one side and a later one on the other side.  I had had several of the extensions saved over the years and had used the best two I had when I put the car together.  Anyway, for me, it doesn't matter ... and I'm not changing either one.
The picture of the passenger side extension is the ‘early’ one, you can see how it folds down a bit, along the light line, towards the grill.  The driver’s side is flat in that area.