Not supposed to dip over rust. Any surface with "tooth" (like bare primer) will make it difficult (but not impossible) to peel off. So, if the car needs bodywork, dipping a car doesn't save as much time/money/effort over a regular paint job. I'll also admit that, in many cases, dipping old cars doesn't look to be as practical as dipping new cars. To me, it seems like older cars often have a lot more textured surfaces and little nooks and crannies that might make it more difficult to peel dip away from trim. But you'd still have to mask/remove a lot of that trim for paint, so you're just shifting the work until the end of the spraying rather than doing it up front.
I've never painted OR dipped a car before. But after watching a few YouTube videos, I feel like I understand the process well enough to give it a try. Like I said, it's not for everyone. If your car is in bare metal or primer, this probably ain't for you. If you want the look of a classic, normal paintjob, or you want to match an original color, dipping ain't gonna do it. But I'm about as far from restorations as you can get.
Plasti Dip Like A PRO! How To: - YouTube
