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The best and the right way to do it would be to cut it out and weld in new steel. The easier way to do it and it probably would outlast you is to get a small sandblaster and blast both the inside and outside of the damaged area. Get some fiberglass matting and a product like POR-15 rust encapsulator and coat the inside (underside) with the encapsulator and then saturate the fiberglass matt and apply it to the underside, apply a few layers, making sure it makes full contact with the underside of the metal and it oozes out of the holes. Apply a few coats of the encapsulator on the top, enough to fill the rust pits. After it dries sand it down smooth.
Just realize this would be a messy procedure, both blasting and applying the encapsulator. You need to blast the metal so the encapsulator will bond to metal. Also, some encapsulators, like POR-15 are a pain to paint. There are other products that do the same and are easier to paint over. Also, don't use a industrial blaster, a cheap syphon feed blaster will work great for this.
Seam sealer isn't going to last for long plus it won't add any structural integrity. The fiberglass would hold for some time if impregnated with the rust encapsulate.
That’s a good spot for a braze repair. My 56 had a couple pin holes under the windshield gasket and the body shop after it was blasted, brazed up all the holes. It’s held up for 28 years this way
I had this same problem except way worse. I sand blasted the area both sides then brazed it all. Ground the brass down and the window gasket and window fit and seal perfect.
I found another picture of the area top center of window frame that I braze repaired. You can see where I started the repair by trying to gas weld it with steel which did not work. The thin metal melted
away before the weld rod would melt putting way to much heat into the surrounding area. This is on my 1949 F1.
Epoxy primer adhered fine to brass. Ask your paint supplier if a product like POR 15 is compatible with your primer. This is a known rust area so any extra protection you can give it I think will pay off in the long run.
Ken
In the original cab of my '49 F-2 it had the same problem but no where near as bad as either of yours but it did have some serious pits and a few pin holes and I brazed over the top and filling in the pin holes and pits. I can't tell you have well it lasted because later on I found a rust free cab from a low mileage fire truck. The original cab had worse rust issues in other parts of the cab. When I first started working in the body business back in the early '80s the small shop I started out in brazed everything, the shop didn't have a MIG welder. We'd make sure the brass were thoroughly cleaned, it helped to blast the braze joints, then we would apply fiberglass filler over the top and finish our repair. Most guys today would cringe at the site of brass being used to do any type of repair because primer and paint have difficulty sticking to it. Just make sure to get it really clean and use epoxy primer. Talk to an auto paint supplier on what they suggest to make sure. I do know some sell epoxy primer in a one time use spray can. I've never used it but have read you do something to the can that breaks a seal between the two parts in the can which mixes them together and then spray it on. Once the components are mixed the primer is only good for a short time before it hardens in the can but if you're only doing a small area it would be worth it instead of buying the full 1 gallon can of epoxy primer and all of the additives.
Since the holes aren't very big once cleaned it you could use jb weld . You would probably need to use some tape or something similar for the first pass to keep it from falling through but it would be every bit as good as fiberglass .
