How to assess rust damage on a quarter panel

No posts in a while, I know. I've been busy with a few non-Mustang projects lately, and this next fix is a biggie - so instead of doing it all in one post, I'll have to break it into a few. Hopefully, this means more frequent posts as well.

I could just as easily call this post "How to face your demons", as I've been dreading this work since I bought the car. Yep, that's right, I knew this car had visible rust on the body and I bought it anyway. I like to think that speaks to my optimism and ruthless persistence -but it could just as easily be a prime example of bad decision making. Either way, I'm about to find out.

Let me orient the reader for this topic. The quarter panel, above, is the expanse of metal that forms the back of the car from the tail panel to the door latch panel. It includes the decorative vent mountings, and forms the edges of the trunk, door, and wheel arch. On the fastbacks, it goes all the way up to the roof line and includes the 'sail' with the fresh air vent. It's huge.

The right side quarter panel had a little hole at the top of the wheel arch, and the half-inch hole went all the way through - not quite big enough to put a finger through. What may not be immediately obvious is that there's actually two pieces of metal here - the quarter panel and the (outer) wheelhouse. The wheelhouse is the big curved dome that the wheel rides in and keeps water and mud out of the car. The quarter panel is spot welded to the wheelhouse where the outer edges meet at the wheel opening.

So, as the title suggests, I'm going to show what I was looking at to figure out how bad the rust really is. I'm trying to avoid cutting more metal off the car than is required to make a good repair*. The quarter panel has lots of curved surfaces and those where the rust and curves are will actually dictate how I decide to patch this part.

*A more experienced person wouldn't have to do this. I'll remind you again I'm not that person.

The easiest area to look for damage is back in the trunk behind the wheel. In this shot, the back of the wheelwell is at the left, and the outer skin of the car is at top. When I went looking, I found nasty looking metal, but it passed the "stab it with something sharp" test, at least where I could reach. But the metal looks like it could be weak and pitted at the edges. Ick.

Next up, I can look behind the interior trim panels and see what the quarter skin looks like at the forward section. Here's the passenger side rear interior trim panel before I took it out (many moons ago...)

I found a monster of a mouse nest behind the trim panel. This is bad, as all this material would be really likely to hold moisture and cause rust from inside the car...

...which is exactly what seems to have happened here. The skin of the quarter is rusted at the wheel well and the rocker panel junctions. So that's a bummer.

When subjected to stabbing test, I found one actual hole an some weak metal that deformed quite a bit. Another bummer.

I then picked out and broke off all the rust nuggets I could by hand, and then hit the outside of the wheel arch with a stripping disc to see what would come off. That half-inch hole became a a 6" x 3" gash through the quarter skin an the outer wheel house. Yikes. Well, just remember, if you can see the rust, it's usually a lot worse under the surface.

Now let's look inside the wheel well. Sticking my head where the wheel usually goes and looking aft, I see outer wheel house is soaked in something oily and has rust at the bottom where it meets the trunk drop-off and the quarter panel. It takes a lot of light and some scraping, but it's there nonetheless.

Looking up at the top of the wheel opening (see the hole?) there is a rust stain that runs the entire length of the outer wheel house from front to back. That's likely a serious bummer as well, as it means that most of the outer wheel house is rusted and will need replacing as well as some part of the quarter panel.

Looking closely at the hole in the top of the wheel arch, the rust seems to extend a good inch up from the edge of the hole in at least two directions.

I decided to cut half of the outer wheelhouse away from the car so I could actually see the extent of the rust on the inside of the quarter panel. After drilling all the spot welds holding the quarter panel to the outer wheelhouse, I marked my line with tape and cut it out. There are also a couple spot welds at the wheelhouse/rocker panel junction.

Stupid sideways pics.

Here's a view from inside the trunk looking forward once the wheelhouse portion was cut out. What's left is just the quarter panel. Notice here the wheel lip on the quarter panel is rusty. It looks worse in person.

Here's one of the pieces of the wheelhouse I cut out. The edge that was connected to the quarter panel is totally shot - it's weak and perforated. Remember, all I could see before I cut this out was the stain in the wheel opening. This is the opposite side of that same piece of metal.

The other part of the wheelhouse was in just as bad a shape. No point in trying to weld to this mess.

Oh, and a bonus - another mouse nest, this time inside the rocker panel. I was going to clean this space out anyways, but now I have to do it while wearing a respirator.

Looking from the inside, the rest of the lip on the quarter panel is just as bad. Again, no point in welding on that mess.

Here's a great view of the rust that is on the quarter skin from the big mouse nest. That metal is weaker than it looks. It has to be replaced as well.

Remember the original half-inch rust hole that we started with? Look how bad it looks from the backside. "You're going to need a bigger patch."

So now that I know where all the rust and weak metal areas are, I marked them all out all the on the front of the panel to assess how much metal had to be replaced.

So, I have a good idea of what's bad and needs to be replaced. And I have a replacement part ready to go. That's actually a replacement panel for a coupe, not a fastback, and that's on purpose.

Stay tuned for the next step - cutting.