roof leaking through rain rail

i recently noticed that after a rain that there was some water inside my 1965 g 10 floor at the back end. upon some detailed inspection i saw that it was running down from the top rear post ..i put my finger inside the cavity where there are wires run..and sure enough it was pretty wet in there...preceded to the outside top at the rear section and noticed that what ever the factory sealed the little rain gutter with had deteriated and after i took a grinder with a wire wheel to it sure enough there were some rusted out holes there,causing the water to enter the inside...so i have grinded all the caulking or whatever it was completly out on the entire top....so what was that stuff that was put in there at the factory and what would be my next step to solve the water problem entering inside ??is ther a product that i can use there that would be a fix or should i bondo or fiberglass maybe ???has anybody out there ever redone this rain rail ????please help im at a stand still....tnx and happy vanning jerry

tnx donivan think i will glass it.i appreciate your quick reply..