Flash Prep
If you're wanting to get the best results out of Flat Matte, you will need to clean the paint with Flash Prep to remove any leftover residues, oils, etc, so that it has a perfectly clean surface to bond to.
Flash Prep is a really simple one to use - with 2 Big Softie cloths folded in quarters to the low pile, spray Flash Prep directly onto the paint surface in small sections (1/4 - 1/2 of a bonnet) at a time. Wipe the product in with your lead cloth - up and down, side to side, so that the product has time to work into the paint properly and remove silicones, oils etc, then give it a final buff with your second cloth. It will flash off nicely right in front of your eyes, leaving a beautifully clean surface for Flat Matte to bond to.