The pneumatic tool is working okay but it would be better I think to switch over to the other tool lying down.  The one I'm using is too powerful for this type of work (without a backing) and is overdoing the chasing.  I'll use the tool with an adjustable air control instead so I can then bring up just the right power setting for working on this piece. After flattening the bulging sections, the next step is to upset the corner area where the two planes meet.  I want a crisp edge here. Well, after getting an initial feel for these air tools to use for chasing operations my conclusion is that they would work fine if the work was backed up with pitch.  If a rubber or neoprene type pad is used on the bench, some rough forming on a flat sheet could also be done using a  pneumatic tool.