I only use tiger hair when the welded areas are down where slush and water off the road can splash onto the back of the panel. Ordinary bondo will act like a "wick" and if the panel is not 100 percent welded, the bondo will pull water thru any perforations on the welded seam, and then it will create more rust and eventually lose its grip to the metal and pop out. The tiger hair is far more waterproof, so on any weld seams that are down in the splash zone, I use tiger hair for the first application of filler. Tiger hair is much stronger than ordinary body filler also, so if its a high crown area or outside corner, tiger hair is less apt to break out.