Screamer,
The product i am refering to is a PPG product called TechniGlaze
The # is DF 715 it comes in a small can & you mix it like bondo, and yes a little hardner goes a long ways.
Good luck sanding that por 15 , once cured it sands lika a rock .......LOL
Por-15 has a primer that you can use first & can be sanded then Por-15 black can be put over it .
Their web site is full of the products they make: http://www.por15.com/

Ah ! But to strive for perfection is the FUN part , and the extra elbo grease needed isnt a waste of time .

On a side note about bondo over led is just like Bob said, bondo never stuck to lead. But my suggestion to remove the lead out of the seam is to get a fire wrench ( gas torch set) with a low heat & melt it all out, or use a small propane blow torch that you use for sweating copper joints to melt it .

The lead now days that you get isnt like the stuff of old as the chemical make up of the lead sticks have changed , I learned how to make it work years ago, and it takes a few tools to to make it easier .
Torch set , lead , lead paddels( of different shapes& sizes), the butter for the paddels to keep lead from sticking to paddels, tinning solution, vixen files (in flat & curved styles).
It also takes some talent to get the heat just right to melt the lead to the panel then smooth with the paddel , all the while kepping a torch going to keep it at the right viscosity to spread.
I learned form a guy that made Darrell Starbird famous, His name was Dave Stucky. this is the guy that made the Lil' Coffin car with that cantelever roof that Darrel said he built.

SprayTech