Lettering on metal door using decanted spray paint

[u/tooloudturnitdown]

Hey! I was looking for advice. I was asked to hand letter on an exterior metal door. But I wanted to use decanted already paint for the letters. I saw this YouTube video where this guy does that technique (https://youtu.be/SEzUAjQeEdI?si=QcTivbMCneSChhAF)

My question is the door I'm painting on is an already painted metal door. How should I prepare the door? Just sand the areas in going to hand letter? Apply a metal primer? Just add a clear coat then letter on top? Any advice would be amazing!