Give them to someone who polishes metal for a living. It'll cost you a fiver, and save you a night's rubbing!
In terms of painting aluminium parts, then they either need to be sprayed properly and professionally using etch primer, paint, lacquer, and then baked with the decals under the lacquer, or alternatively powdercoated with decals on top. Powdercoat is super cheap, and dead easy to get powdercoated again if it scratches in the future. Decals would be placed on top, and just keep a few sets spare in case you damaged the decals.