Check into Kustom shops pre flattened single stage. There fairly affordable and durable. My experience is flattened paint is impossible to keep clean. Every fingerprint shows up as a greasy spot not to mention road tar. Also minor scuffs leave glossy marks. Primers do not have the required uv protection and any sanding a polishing leaves a glossy finish. Flattened single stages and clears can be polished but all leave a low gloss finish instead of a flat finish. The only true flat paint I have used that maintains its finish is Alsa corps soft feel paint. Just as it sounds it feels like velvet and maintains a true flat appearance. It is very expensive but works well on interior stuff because of its durability. Its like 350 for 1.5 gallons rts versus custom shops 100-150 for 1.3 gallons rts.