If you're wanting a quality paint job you need to remove all panels in order to do the proper prep & paint. Leaving the fenders on will allow any rusting areas on the mounting flange(s) to keep rusting. I've seen a lot of folks cut corners & leave panels on to save time and money but IMHO it's a substandard paint job. To answer your original question it's often easier to do the straightening while the fenders are on the truck then remove them for paint. If the truck didn't come from the factory with welting there's really no need to put any on unless you just like the look.

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