Painting a garage door is not a simple operation. Most garage doors these days come ready primed so if you are painting a new door it will be a matter of applying (preferably) a proprietary garage door paint to the primer with however many coats are recommended on the tin. If however, the door is unprimed and, as most of them are galvanised to stop rust, the procedure is slightly different.
Just like trying to paint ceramic tiles, normal paint will not stay on galvanised metal for very long at all. If you try, it will look good for about 4 days before it starts peeling of in huge flakes.
Degrease the door with sugar soap then allow to dry. Apply a metal primer before applying your garage door paint. If you are over coating a previously painted garage door then a light sanding is required along with the same degreasing process. When painting metal it is really important that any loose paint is really well rubbed down and if, in the process, you get back to bare metal, then this metal must be primed. It is never a good idea to put a top coat straight on top of the old top coat so use a suitable undercoat first. Remember the better the job is prepared, the better it will look and there is nothing quite as scruffy as a peeling, faded garage door. Many companies make a metal primer and undercoat mixed
Metal garage doors can be painted by brush or by roller but if using a roller remember to buy one with natural fibres rather than a man made material. After rolling, for a top class finish, run over the paint with a good quality, dry brush. This will "polish" any roller marks out and give you a lovely shiny finish.