Take your time with it if you don't have a shrinking disk get one and watch a few videos on how to use it properly.
Do not get excited and start pounding on it!!!! There is a lot going on with it and previous work which was not done correctly. get some Dykem or machinist Dye, a good flat Rasp file and be prepared to take your time Hours and hours to find Hi and low spots first.
Coat bare skin with the Dye then run file lightly over skin to identify hi and lows.
You use the file just to remove the Dye never to remove metal.
Just as a warning & description of how panel straitening takes place:
A Low spot over a foot or two feet away from main damage can lock a panel to coming back into shape in main area you are working DO not concentrate your attention to just the damaged area BUT the whole panel as a whole.
ALSO never work a dent at its lowest or highest place!
ALWAYS work damage from Outside in! NEVER at center of damage out! ALWAYS work Outside IN.
It is my opinion to always work low spots first before working the Hi spots, Avoid Hammer on dolly when working low spots as this stretches skin, always hammer off dolly AND NEVER hit the panel Hard!!!! Push up on low spots with dolly firmly to hard and TAP skin OFF dolly with Low crown hammer around the low spot to raise the low spots.
Repeat the Dye and file step keep working all low spots first when you got all of them then use the shrinking disk on Hi spots This will more than likely relax the skin were it was stretched reveling new low spots! which will again need to be raised and the process will repeat. Never over heat the skin You only use the Shrinking disk for friction to heat the panel enough to cause steam when cooled with a wet rag, never enough to get red hot Never enough to cause Blueing.
It just depends on how bad the skin was stretched by who ever did the work before as to how many times the process will require repeating.
I have worked a fender for DAYS taking my time repeating the process.
It is very possible to OVER work the skin and cause cracking of the skin so your work with the shrinking disk needs to be methodical as possible its not a magical eraser that you can just keep using over and over as long as you wish the metal wants to go back into shape it was stamped too it just needs gentle help to get their.
I strongly suggest getting a Old hood and or an old fender damaged ones are great and PRACTICE on them FIRST!!!!! If you get undamaged thats great too in your case go to back side of practice panel and put some big dents in it then practice techniques described too straiten it before attacking your project car.
I strongly suggest Getting a Book called "The Key to Metal Bumping" its small and less than 50 pages and costs less than $20 when I got a copy, The techniques explained in detail are invaluable to anyone doing auto body metal work.
https://www.amazon.com/Bumping-Instruct ... 350&sr=8-1https://www.eastwood.com/wolfe-s-easy-s ... -kits.htmlThese are my favorite and go to tools when working dents, you will be amazed how much can be done with just these and a shrinking disk and a good Rasp file.
https://www.amazon.com/Martin-MRT158G-G ... 333&sr=8-1https://www.amazon.com/Martin-1057-Ligh ... 291&sr=8-3https://www.amazon.com/Martin-1059-Heel ... 291&sr=8-4