Our cowl is shown here properly repaired and basking in a coat of POR-15 rust-preventive paint. Using POR-15 is a sure way to prevent ever having to replace the cowl again.
Cowl vent leaks are the Mustang owner's worst nightmare. Just about any vintage Mustang in need of restoration is going to need some sort of cowl panel restoration. Some cowls have minor leaks and can be patched, others need complete replacement panels (and we can thank the reproduction metal companies for making this step much easier), and the worst of them need complete replacement cowls.
There is very little in the way of diagnosis-simply run some water into the cowl vent and see where it leaks from. On the worst cars, you can also see rust damage from under the dash-with the heater case and the driver-side fresh air vent removed. It's pretty much a given that the cowl will leak when you test it in this manner. The only things left to do is remove the top of the cowl area and survey the damage.
1 We're starting this project in the middle of a complete restoration, but if you're repairing only the cowl area, you will need to remove the windshield, the dashpad, the hood and its hinges, both front fenders, and the wiper arms for access to the cowl's spot welds. Clean away all old paint and seam sealer with a wire wheel.
But why is the Mustang cowl design so prone to rust perforation? Well, it's a simple fact that the cowl of the early Mustang was poorly designed. The cowl, as it was welded together, had no access for proper paint application or rust inhibitors. Back then, metal wasn't dipped in EDP primer as it is commonly done now on modern cars; the protection came strictly from paint application after the body was assembled.
The drainage of the cowl was poor, and any large items, such as leaves, twigs, and paper, could easily slip past the large cowl vent slots and dam up the drain areas-causing standing water to sit around the unprotected metal "top hats" of the cowl vent areas. Though the design changed for the '69 model, it's interesting to note that Ford used a similar design for the Fox-body Mustang from 1979-'93-a large cowl vent opening with a single air inlet on the passenger side. The drainage was improved, but the vent slots were plastic and smaller (allowing for paint application inside the cowl before the vent was attached), and the metal was EDP-coated as well.
Project '66 needed cowls, of course, as do most '65-'68 northern cars. We didn't know what we would find-and except for looking under the dash (the interior was gutted for paint and body work), we didn't see any major rust perforation. We hoped cowl patch panels would complete our job efficiently. To be prepared, we ordered a left and a right cowl patch panel from The Paddock and sent them over to Classic Creations of Central Florida, so that when we arrived to shoot this article, they would be ready for assembly. Take a look.
2 Drill out the spot welds that attach the rear fender apron extensions to the cowl. Be thorough to ensure you have found and drilled through all of the spot welds. Drill through only enough to separate the spot welds--you don't need to drill through completely. | 
3 Using a hammer and a chisel, separate the rear fender apron extensions from the cowl. We opted to purchase new extensions since ours had seen tough times throughout the years and were damaged upon removal. | 
4 Once the fender apron extensions are removed, you can start on the almost 200 spot welds that hold the cowl tank halves together. Classic Creations of Central Florida's Danny Gaydos prefers to start at the windshield opening, but it's your choice where to start. |
5 To access the spot welds across the front of the cowl, Gaydos stands in the empty engine compartment. You can reach them from the side of the car as well. | 
6-a After drilling out all of the spot welds (look again because you probably missed one), use a hammer and a broad chisel to separate the top of the cowl from the base. | 
6-b Work your way completely around the tank, searching for any missed spot welds. |
7 With the cowl halves separated, the top half can be carefully lifted off and set aside. Again, make sure there are no hidden spot welds that were missed; otherwise, you will bend the tank top during removal, which will require more body repairs. | 
8-a This is most typical of a northern car and the condition of the cowls. The driver-side cowl hat has completely rusted away, causing separation between the hat and the cowl. The passenger side isn't much better, although it is still solidly connected to the cowl. | 
8-b We're lucky, though, because patch panels will fix this problem. Anything worse and we would have had to scour salvage yards for a good cowl assembly. |
9 After cleaning out the leaves and other junk from around the wasted cowl hat, Gaydos proceeds to cut away the damaged metal, taking into consideration the size of the replacement patch. | 
10 Working on one side at a time allows for the most amount of strength to remain in the cowl area to prevent misalignment or warpage. The left-side cowl hat area is now free to be removed. We now have only good metal to weld to. | 
11 To prepare the good but surface-rusty cowl area for the new patch, Gaydos uses a grinding wheel to clean the surface around the fresh cut for proper welding adhesion. |