Jim,
Sheet metal most of the time is hot rolled. You can tell because it is a darker color because of the mill scale (kind of like a black layer) left on from the heat.
If colded roll, which usually is for specific applications the sheet metal will not have the mill scale.
There are a couple processes that they use to remove the mill scale.
When I buy sheet metal I ask for the Pickled and Oiled (P&O).
This is hot rolled sheet metal that is brighter in color has a thin layer of oil to prevent rust and you do not have to remove the mill scale (the pickling process removed the scale) if using a patina.
Depending on the layer of mill scale the torch will make different heat patterns.

warren
http://www.metalrecipes.com