Etch primer is meant to be what the name says, that is you cannot paint over a mirror or shiny surface, the purpose is to etch the surface, to a suitable surface profile so the paint will adhere. So if your aluminum has been out in the weather, the acid and oxygen in the atmosphere will have already etched the surface i.e. it looks matt not shiny, if its shiny it needs an etched surface. De-greasing would be the main object then. As for the paint its what you can get at the green shed, your tools will soon chip the paint anyway.