Cuz it's quick and easy to work with. And by the way, take a
stick of body solder or any solder and bend it back an forth,
then say it won't flex.
Bondo or the modern equivalent to it, is great stuff and it is
much easier to slab some on a low spot and shave it off and add
layers to it, but I will still use lead in an area of high
stress.
Here's is a section at the nose of the rear fender, where it's
bolted to the running board, that many of us find stress
cracks. The cracks have been drilled and welded and the repair
is tined and ready for lead. I wouldn't trust plastic as a fix
in a spot like this:
http://www.pbase.com/dennygraham/image/114604993As far as the seam that we have been talking about on the side
cowl joint, after that seam is spot welded back properly I
would not expect there to be much flexing there at all. Even
though that particular seam was leaded at the factory I would
probably elect to use a plastic filler there also, as I would
for a small shallow in a fender, door or other low stress area.
Denny Graham
Sandwich, IL