[jpnovak]

The only way to flatten this is to shrink the metal. There are two ways.

method 1. Go buy a shrinking disk. I have not used one personally but know someone that has with fantastic results. Think of it as a grinder that does not remove any metal. They are usually a concentric cut stainless disk. You bump low spots out from the back. The disk only touches the high spots. The disk will heat up these peaks of the metal and then you shrink them by rapid cooling, Usually by application of a cold, wet rag.

Do you have an acetylene torch? If so you can do the same thing with method 2. Heat up the stretched area and then shrink. The rapid cooling will cause the metal to contract into its original size and shape. I usually start at the perimeter and shrink toward the middle. For smaller area or more cooling power use a can of spray duster. It comes out very cold, especially if the can is upside down. :0

Take a hammer and bump out some nubs on scrap sheet metal and then have a bit of practice. you will be amazed at the results.