Method Filling Small Rust Holes?

He might have been using lead. Eastwood sells kits to do this: one is lead and the other is a newer product without the health risks.

It's called 'wiping' a seam or whatever you are soldering. When I was a cable-splicer back in the 70's we had to do what we called 'pot-wiping'. You had a bucket of molten lead that you poured over the end of the lead sleeve and wiped it into shape with an asbestus glove on your other hand. Made a very nice looking bullit-end shape. On a car you would heat the metal with a fan type fitting on the torch and fill the seam or hole with sterrine solder. After it cooled you would heat it again and wipe it with a tool similar to what they now use on bondo to smooth it to the shape of the car.

The process of using a BUNCH of lead to customize cars in the 40's/50's lead to the coining of the phrase "LEAD SLED".

miek and dave are right, likely he used lead as solder itself is tougher to control than lead is even though they are similar materials.