The bottom line is that we would strongly discourage any consumer from painting their garage door.
First off, most manufacturers will void any applicable warranty if an applied finish is not carried out by the manufacturer themselves. Secondly, it is common for consumers to apply incorrect types of finishes which can cause the paintwork/primer to peel, bubble or disfigure the door, revealing the door’s material composition.
The doors best suited for painting are metal doors. The majority of metal doors come with a minimum of a primer finish making them suitable for painting; however, most people leave them as they are.
Wooden doors can be painted or treated as required, yet it’s always suggest that you follow the manufacturer’s recommendations/instructions.