This is a comment from someone who is "learning on the job". I straightened out my old front fenders to see how close to proper form I could get them. They were pretty trashed and bent up but I managed to get them back very close to the proper shape. I ultimately bought some used replacements because the old ones were too far gone, rust wise.
When a fender buckles perpendicular to the impact force, which is very common, the metal cannot be straightened by simply hammering at the crumpled area. This causes further damage and distorts the metal in a different way. The fender has to be pulled out the exact opposite to the way the force was applied. At the same time, the crease can be worked on to allow it to move.
I found that the metal seemed to have a "memory affect" and the closer it got to the proper shape, the more stable it was.
