Hi ZipZap!
Try Solver Paints!
http://www.solverpaints.com.au/In what seems like a lifetime ago I used to be a Painter and Solver was always my choice of materials for the quality and for the service.
All roller doors have grease on them at the edges, so what you do is to wash down the roller door with Solprep Degreaser with a Green Scouring Pad. you then wipe off the used degreaser with a clean rag to take away the contaminate. The scouring action of the pad roughs up the surface of the powder coat which enables the topcoat to stick. Dont use Steel Wool as it drops bits everywhere that rust later on in the Top coat!
For the top coat you use Two Coats of Solver Duragaurd Satin Finish, Satin because it is the same sheen level as brand new Colorbond Steel and give the right finish. Gloss just doesnt look right and Low Sheen is better for masonry walls and stuff of that nature, it makes Colorbond look to aged which isnt likely to be the look you are after!