Just a thought, if said garage door is galvanised, as many are, you will need special primers, and possibly mordant solution. The primer and undercoat are a critical part of any paint system. Studying the chosen manufacturers technical info is well worthwhile.
Never be tempted to use boat paints intended for use on steel, on a wooden structure. If metal, then fine. Wooden things need to breathe, hence the existence of microporous paints.
Final thought, just don't ever, buy paint from a DIY shed, its cheaper for a reason. Only ever use "trade" paints, but make sure you have read their technical info first.