If you need to touch up failing paint on anodised aluminium, there is a critical issue to understand before you rush into the process. Metallic surfaces are smooth and nonporous. This makes them ill-suited for paint adhesion. Although anodised aluminium is rougher than ordinary aluminium, it may still reject new finishes unless it is pretreated with an acidic primer. If the paint on your anodised aluminium is failing, it probably wasn't properly applied.
Remove as much flaking paint as possible from exterior anodised aluminium, using a pressure washer with a low-pressure tip. Wait two to three hours for the surface to dry. Skip this step if the surface is indoors.
Scrape off remaining bits of flaking paint using a putty knife.
Smooth the rough edges of chipping paint using low-grit sandpaper.
Place a dust sheet beneath the anodised aluminium.
Coat exposed areas of bare anodised aluminium with galvanised-metal self-etching primer, using a 5 to 7.5 cm (2 to 3 inch) latex paintbrush. Wait four hours for the self-etching primer to dry.
Wash the brush with water.
Apply touch-up paint to the anodised aluminium, using the clean paintbrush. Wait two hours. Add more paint if the self-etching primer bleeds through.
Although you can use most any type of paint on properly-prepared anodised aluminium, enamels will prove more durable.
Do not touch up exposed areas of bare, unprimed anodised aluminium or flaking may ensue.