You know how the paint starts to fade with the weather, especially on exterior doors? Here in Florida it gets really bad. No shine will be left on the surface and the paint will start to have a whitish discoloration from oxidation. I discovered, quite by accident a very easy solution to this problem.
Lemon oil, the really cheap stuff, under any brand name. The package will say furniture polish. None of these are actual lemon oil. It is a petroleum based oil with lemon fragrance. No other type of furniture polish will work for exterior doors.
The best rag for this purpose is an old sweat shirt cut up. My reasoning for this is that the fluffy side can be used on metal and the flat side is used on wood. When wood starts to get old its fibers start to split, a rag with loops will catch on these fibers and create a mess. I learned this the hard way by polishing with a green fluffy rag. It was near impossible to pull out the green fibers that got stuck in the wood.
Try to use just enough to create a shine without leaving a thick layer of oil, which will leave too much of a residue. You do not want accidental touches of the door leaving clients hands greasy.
Painting metal doors is a specialty and due to specialization it is costly to repaint. This treatment saves your client money by avoiding a paint job. In addition to this, one treatment will last weeks. Another nice benefit.