I've tried googling and haven't found any answers, but has anybody tried to fiberglass the cowl to prevent future problems with rusting? How does water affect fiberglass? I'm not experienced with fiberglass at all so please forgive my naiveness.
If there is already rust there it will only be a very temp fix before it starts lifting off. If going over clean rust free metal it will bond very well and be water proof but will still eventually fail. Fiberglass repairs just don't last when glassing to metal but if its the only fix you have then prepping the metal is very important. It has to be clean, dry and rust free or at least a rust converter used before hand. If your only dealing with pin holes you might be better with something like POR 15 or another rust encapsulating product. A couple of coats will seal pinholes and bond very well to most rusted areas. It dries to a rock hard finish and lasts a long time.
Like tos said ,fiberglass and metal would not be a good mix especially in that area .It would not take long to lift and fail.
Where exactly is the problem any pics ? There is a plastic repair piece for the opening under the cowling inside the car ,if the damage is not real bad the rot can be cut away and when sealed well they will last several years .
I know this is not what everyone wants to hear. On one of my old cars, before I had a welder I did cut the cowl vent panel off to access the hats. Both hats had completely rusted away. I taped the holes from underneath and laid 2 layers of fiberglass over it. I did this back in 2004 and it has held up fine, no leaks. While I was there I made the wash-out openings on the side larger to allow things like leaves to wash out easier. What I do miss, since I sealed the hat openings, I no longer get the fresh air, which sucks on a hot day. It was a factory heater delete car so no defroster either.
Funny thing, after I was done, I had a shop tack weld the cowl vent panel back on.