The situation is quite simple, I've got an awful rust spot to the left of the left-most heater vent area. I'm wondering what ways I can get to it best. My hope is that I can remove the fender, and possibly a piece under the fender in order to expose this bad patch from the outside. So why do I ask instead of just taking things apart to find out? Well all the fender bolts are very rusty, and I've broken a couple off already. Don't really want to go down that path if there is no point.

Heres a super close up of the damage:




Heres a view of the whole area effected:




Heres a view just so people can see where that is in relation to everything else:



Probable cause is still to be determined. Windshield leakage doesn't look like the issue at this point. Likely something to do with the wipers flinging water, or a drain back-up in that area under the vent panel. Probably some thing as simple as a spot that didn't get paint.

 

The only thing under the fender is the structure which is all spot welded together. To go any deeper beneath the fender you have to drill out spot welds. You can search some of the build threads and see what it looks like under the fender area, and you can look at Aaron Cake's Cosmo restoration videos on YT. He takes the fenders off and repairs some of the structure underneath. If your fender bolts are rusted and breaking there's a good chance that piece of the structure under the fenders is rusted too. Aaron Cake replaced that piece on both sides of his Cosmo. Neat stuff in his videos.

Looking at your pics and looking at the donor car section I have, it looks to me like the problem is from a common rust spot on the first gens. For some reason the area around the hole for the drivers side wiper arm rests low and water accumulates there. I think your rust was caused by water leaking around the wiper arm and if you have AC, maybe that may have contributed if that colder drier air mixed with warmer humid air coming in through a leak - maybe. Here's the really bad news: the metal in that area is all welded together. You'd have to cut that stuff out and fab or obtain a rust free section. Nothing unbolts from around that area. Now I'm worried that mine may be in the same shape!

 

Damnit...I was afraid of that. Indeed, I do follow Aaron Cake's videos at length, and know exactly which one you are talking about. That is sort of what gave me the idea. I figured what was under the fender was likely similar to what Aaron was working with.

Well, I think this may be the last straw... Time to buy welding gear and get cracking. Grrrr...

 

Yep, that's pretty common, and in my opinion..terminal The rust seems to begin deep inside. I think possibly from water driven into the A pillar when driving, into areas without rust protection.
Here is a photo of a terminal patient with observation holes cut through the top. Note the pile of rust accumulated in the virtually inaccessible area.
The photo from the cowl area shows the enclosed part that is the second layer seen from the holes cut from the top.
I had a pic from under the dash but can't find it. It is like yours but not as extreme.
Too many layers of metal from different angles coming together in inaccessible area to try and weld.
If you tackle it my hat's off to you.

 

cut off what you can with a sawz-all or the like and then bend some strips of 1/4" thick mild steel to "bridge the gap" in different directions to tie everything back together. try to determine the point of entry for the offending water and seal that. Don't close the "rust hole" up completely so next time you'll know if water is seeping behind your dash (i.e. wet carpet)...

p.s. if your building a show car and people are going to be poking around with mirrors then please disregard...but if you just like driving this thing for fun I think the above repair will suffice.

 

You know...I think You might be onto something with that viewing hole. I will indeed be tacking this. Unfortunately I'm in sort of a tight spot; everything except a few spots on the body are in pretty good shape on this car. I'm too far in to scrap the whole project, I'm too far/right in the middle to turn around and sell it (or at least not loose a huge amount of money), and I simply don't have the time or space to do a tub swap. I see my only option at this point simply being to tackle the project and see what happens.

Yes, point of entry will indeed need to be found. I know a couple people who can do some welding for me, so I think I'm just going to tear it apart and see what I can brew up.

The above repair will absolutely suffice; this car is to be the exact opposite of a show car. I simply plan for it to take over the daily grind and occasional auto cross grind.