The Old Gunslinger does make very good point about making sure there is not something underneath the skin holding it up.
Back to "Metal Shrinking" If you only need minor shrinking or are unsure of yourself, leave the wet rag off of the procedure. I rarely use the wet rag. It works for sure but you will get shrinkage with or without it, just not as much. Also it will not temper the metal as much and make it stiff.
Also keep in mind. Shrinking when overdone, or improperly done, can create all kinds of other problems. It can pull waves across the panel and cause stress that can make low spots worse if you have not correctly "evaluated" the panel.