I got the car back today!
The repair is fabulous! All new smooth black metal where before was only rust, and shame.
he also touched up multiple 7yr old bicycle rider marks, and a billion rock chips, etc etc. and buffed the bejeesus our of the whole thing.
They lay down a layer of the spray weld (sprayweld, spray-weld), then cover that with a thin layer of putty, then finish that. It seemed very cost-effective to get the rust fixed this way. Anywhere that I can insepct directly the rust is GONE, it is all new metal.