i myself am gonna carfully braze in hand formed pieces for the big areas and then lead the poop out of the whole area...my bodywork friend advised me that that is the best way if you are not doing a full resto,and from what i know...lead dont rust
my rear window was starting to do the same but i stopped it before it got bad...just gonna clean the crap out of it and throw some lead in there too.....thank god im not the only one that is going through this