Prostor awning end caps.

I have a Prostor awning on a Sussex Duo. Mechanically it seems fine but the paint on the end caps, both on the roll out section and the section fitted to the roof has peeled or is peeling off. Having removed one of the end caps it does appear that the problem is purely cosmetic. I believe that I can purchase replacement parts but things being what they are I was considering repainting them. Therein lies my question. What with? The caps are originally white and I have both Rustoleum and Hammerite white paint available but was wondering how they would work on the cast alloy of the caps. Has anyone any experience or thoughts on the matter?

Thanks

The problem of how paints react to alloy was my main concern, so your suggestion of the primer for Hammerite is certainly of interest. 
Thank you.

If painting aluminium, there are several unbreakable rules!!
1 Get it to bare metal, and I mean bare
2, and far and away the most important, spray with several coats of ETCH PRIMER, I use UPol from my local motor factors, any etch primer for metal will do tho
3 Let that set 1 hrs (it'll be touch dry in seconds) then spray with whichever car white or colour you choose.IMHO, this is not a job for Hammerite.
4 Leave it all alone for 24 hrs, then spray with several coats of CLEARCOAT, same car shop, leave another 24 hrs then refit.

If you do the above, they'll be better than new!

Thanks very much for that.