if you weld it solid, you should see no line from the welding. I seal all mt welds. I use either epoxy primer or Master Series urethane to seal the welds; best if it can be done front and back. Then I use, most times, regular filler. I have used short strand fiberglass filler, just depends on the area, as my first coat. You want the filler to not have any chance of getting wet. The fiberglass filler does that. But also, sealingthe weld with the primer does the same thing.
Mustang RUSToration & Performance