The application of lead is actually an art-form. However, the modern resin fillers we have today would have been snapped up by the majority of old time body men and I'm not talking about "Bondo." If you've ever used lead, the early resin fillers, and finally the newer products from Evercoat like Rage and Rage Extreme, you'd understand why. No filler, modern or old school is going to replace quality metal working. However, the new stuff is so highly formulated that all of the negative factors of "Bondo" have all but been eliminated. The Evercoat products are by no means cheap but the stuff is so easy to work that the $30.+ a quart can be made up in not having to change sandpaper every 5 minutes.
If you haven't tried Rage or Rage Extreme.....and you're still trying to get decent results from "Bondo"...do yourself a favor and move up to a truly 21 century premium product. You'll never go back to the old ways.
Dave
