2nd gen F bodies are known for poor cowl drainage. They suffer more if the car sits out in the weather for extended periods without moving like mine did. Puddles form on the top of the cowl by the ends and GM made a nice place for a little pond to form inside the cowl right above the brake pedal. About 10 years ago I fixed the inside pond part and on the top I had cleaned what I could get to (with the fenders still on the car)and used rust converter followed by some bondo to fill pits and semi gloss black paint. Since the car doesn't get wet it's been fine but this time I can do a better repair since the fenders are off.
Although both ends on top of the cowl had pitting and some rust through the drivers side was worse due to the cowl tag holding water under it so I'm showing the repair on that side but I did make a similar panel for the pass side. Since they rust from the top down due to the puddling we're seeing the worst of it. Replacing the entire top of the cowl can be done however in this case I didn't feel it was justified. The center of the top of the cowl is in great condition and this is more of a track car that is street driven so strength and fire protection are my goals on the cowl/firewall area. I could cut and weld new metal in the ends but I felt this would be stronger and easier.
The top of the cowl had already been wire wheeled and the center section primed to prevent rust. The raised area around the fender mount was thin so I cut it out around the fender mount then wire wheeled the area to clean metal.
Made a pattern out of paper, transferred to sheet metal and started bending, hammer/dolly, etc. into shape. Made the raised area for the fender mount with a big impact socket and a die handle with a BFH. It's humid here in So FL so everything rusts as you can tell by the sheet metal. Not to worry though it gets cleaned up well on both sides before final install.
Once the panel is shaped the edges are finished and it gets cleaned. Then it gets marked where the soft areas are in the original panel and screwed into place staying out of the soft areas. Then the screws are removed.
Next a thin layer of panel bond is spread on the area where the panel will go. Bonding the panel onto the existing panel is much stronger than replacing the original metal with a single layer regardless of whether it was bonded or welded in place. The panel bond over such a wide area is stronger than mig or spot welding without the warping problems. For those who avoid panel bond because they don't want to buy the expensive double barrel caulk gun you can push the 2 parts out evenly (without a mixing tip on) with a piece of pipe if you remove the back of the label so you can evenly move the plungers so you have equal parts. Then mix like bondo and spread where you need it with a plastic bondo spreader.
Once the panel is in place install the fender mount bolt with wide washer on it and screw the panel on, then do a finger smear around the edges with the panel bond that squishes out which will seal it so no water can get under. Make sure the drain hole is open and that the panel bond will keep water from getting between the panels there also. Because the fender mount is high there's no need to have that bolt hole sheet metal sealed (and you do not want a washer bonded there!)unless you're gonna throw the car outside for 10-20 years.
Once the panel bond has set remove screws and fill the little screw holes with panel bond. A piece of tape underneath will keep it from dropping through till it sets. After the panel bond is fully cured (24 hours) sand the panel bond and apply a little plastic filler then sand. With the repaired area done prep the whole area surrounding the repair and prime. Before priming I used a large drift and a BFH to make some spot weld looking dimples along the cowl lip so it'd resemble the original part in the center.
