so just putting some fiberglass over the weld seem(lap weld) is not good enough, i should use lead AND fiberglass, isnt lead bad for you and bad to use/breath :-|
Actually, you shouldnt need to use any fiberglass. Just Lead followed by the body filler. The process is much like sweat soldering a copper pipe or soldering an electrical connection. You're just using a larger block of solder and a larger heat source. You can find lead , the tools and an instructional video at www.eastwood.com I bought the middle of the road kit.