Clear coating of any kind over polished aluminium is a complete waste of time. However if customers insist, then I doubt very much anyone doing that sort of work commercially is going to refuse to do it? Powder coating applied over bare metal may well also fail after a while. Proper way to coat a frame is to blast clean, heat in oven to degas, then apply epoxy powder primer, and finally top finishing coat. A lot of people miss out the epoxy prime coat, and this can lead to problems on bike frames and parts.