removed my vinyl top and found this

They used plastic filler(bondo) when there would be a vinyl top covering it.Lead filler was used if the roof was painted and had no vinyl top.This was the way the factory did it back in the day.I would shot blast the rust out so it does not come back and then use filler to smooth out the seam.You must take care of the rust by media blasting or using some form of rust eater or the rust will come back.