[OldMav]

If you are going to powder coat aluminium the metal has to be chromate treated and preferably acid washed first. Powder coating is a porous coating which allows moisture in to start off that white corrosion process which will bubble the coating rendering it useless in a very short time if open to weather. Absolutely useless around salt water even when treated properly.

For a alloy tray if you must paint it then a acid wash, then a chromate wash, (Alodine or Irridite) then a epoxy primer and a mastic type (Rhino) colour coat would be the most durable. But any of the marine colour coats are very resistant to bumps and scratching and usually cheaper and a hell of a lot more durable than limited solids car paints.

Edit: nothing will bite into the extruded mill finish on alloy even those etch primer paints for alloy will not bite into an extruded mill finish you have to sand it first if you want any sort of surface coating to adhere to aluminium. The industrial Acid washes with nitric acid will remove the mill finish even then sanding is best, those Ali Brite cleaners with hydrochloric and sulphuric acids will not remove mill finish, you need to sand it first.. A fine garnet whip blast work well as does soda wet blast..

 

[OldMav]

As said aluminium oxides on the surface is not good for any paint process especially powder coats which use a mechanical bond process for adhesion.

You could sand and acid wash and chromate treat the tray yourself then give it to a powder coater to be sure. Because from experience without that process to remove the oxides and seal the surface with chromate, paint or powders have a hard time staying on Alloy.

A link for you to check on my advice
Aluminium Powder coat

Just to be a bit picky most coaters dont understand alloy. after acid wash you have to chromate before the surface is completely dry otherwise the surface will start to oxidise again and the normal chromate wash isn't as effective. For alloy boats like my avatar we use a bromelain mix in the chromate to counteract oxide development. Or just dont acid wash in direct sunlight.