Hey Evil. Both products worked really well. VHT does sell a product for prepping the material, however we did not buy/use it. It worked well on the armrest without it, however that's the only thing I've done with VHT. For the door panels, I would use the SEM products. It really does a great job of removing any grease or dirt and dying the panels completely. At the end, it really looks like you've done a quality job.