I'm plowing through a restoration of my 83 Zee... I've owned this car since new... One owner car.

In the late 80's I had a cracked windshield and they did a terrible job... scratched the factory paint and rust stared soon in those scratches.

Move ahead to the late 90's and I had the car repainted. The body shop said they Re&re'ed the windshield to fix the rust... I'm not so sure they did.. Regardless the rust around the windshield returned.

The car sat outside for about a decade waiting for a honest restoration.

It is clear that while it sat, the windshield had started to leak.

I have pulled the windshield and I discovered that all of the butyl goo was slicking to surface rust and not to the car.. Leak explained.

SO I start to have at the window frame with the angle grinder and a nasty wire wheel. Red rust was easy, cleaned up nice... But there is a coating of black ROCK HARD stuff... What is this stuff? Magnetite? aka Fe3O4 aka Black Rust? Wire Wheel doesn't touch it, sanding disks, barely makes a dent. Anyone experienced this stuff? Any tips on dealing with it?

In other news, other than this problem the car is remarkably rust free. The only victim of the leaky windshield was the "Stainless Steel" Vin plate which has to be the worlds lousiest stainless... It is badly rusted and perforated... one of the numbers is gone and 2 of the remaining are barely there...

Any advice on dealing with the black stuff?

 

Spot media blast it, or cut it out and weld in new.

 

Update, I had my body guy look at the windshield frame and we had a general discussion about black rust, red rust and media blasting. His advice.
1) Only cut out metal that is perforated or so thin to be useless... Mine is not in that condition
2) Media blast as a last resort. Even with masking and a real focused effort media will be everywhere and will be a bear to clean up and remove.
3) Both media and cut and weld done by him would cost more than the car was worth.

He added that Soda will remove light rust, the Red Stuff but will bounce off the the Black Rust.

Then he offered a DIY option that I tried last night and it worked amazingly well.

His cheap at home solution was a $8 bottle of acid toilet cleaner from HomeDepot there are various brands all are a concentration of HCL (Hydrochloric Acid) the one he suggested was 10% though 15% and 20% are available. We ripped long thin strips of an old beach towel soaked it in the cleaner and then laid those in the window channel.

Let that sit for 30-45 minutes... Warning, regardless of what the bottle says, the stuff really smelled AWFUL. Wear a respirator, rubber gloves and eye protection.

After the half hour (give or take) The black rust literally wiped off, all that remained was silver rust free metal (slightly pitted).

We neutralized the HCL with a water flush and then baking soda.

This morning it was still perfect. Modern living through chemistry