I need rust help!!!!

[DOIBUGU]

here what im wondering my 70 cab has a roof skink and an inner roof skin its not a headliner truck i have rust in between the two who do i stop that is there something i could do like pore someting through a whole and fill the gap like pro15 wait for it to harden or dip the whole cab ( i seen it in a show once where you charge the water and stick a rod in to attract the rust not sure what the method is called but maybe) i dont know i just dont want to do a **** load of body work to have the rust eat the new paint in one show season and have to repaint the other idea i have is to just cut the roof off and make it a roadster