For months just do it when you wash your car and it's amazing been doing mine for the past six years still looks like new.. It's all down to the water marks on the plastic coating first few times I did mine I used bottled water as it has no limescale or chloride that makes the mark's....
Hope this helps you out
Drive safe and stay lucky

Les n Co
I found the one that is going to dethrone that.

As you know, I have the Autoglym trim from your suggestion and I was absolutly sold on it. It was my go to. That problem was fixed.
But I wanted something to keep my tires shining. I search for the best product out there and found...
Perl from CarPro. It's a very interesting product. You can dilute it 1:5 and use in the interior to protect plastics, even leather. You can dilute 1:1 or even without dilution for outside plastics and tyres.
So I did. Both.
The interior works amazing. 4 months of UV protection from what I read, and fact is that two months later, it's still going. Repels dust and stuff. Makes everything beautifull.
The outside, I used without dilution on plastics (all look great but to keep them protected) and I used it on the tyres. The tyres surprised me alot because even after washing the car two weeks later, the shine is still there, doesn't go away or grab dust like other products.
Fair to say that I was amazed by this product. It's, without a shadow of doubt, the best car product I ever found (and I use alot of Meguiars good stuff, like Hibrid Ceramic, Glass cleaner, etc).
So, the other day, I was thinking about the aluminium arches and remembered what you said. "It's aluminium with plastic". So wait. If the Autoglym is for trim and plastics, it's actuating on the plastic more than the aluminium, that's why it looks new. So, I have this product for plastics, that does all that Autoglym does and last alot longer, does it work on the arches?!
And the answer is... YES! And it works like in the tyres! It lasts forever! You can wash the car and the arches keep looking new, no spots, nothing! 4 weeks now since I tried it and it's not perfect, but still looking very good! The autoglym by now would have been completly gone (remember I live in sunny Portugal).
So, dear Obi-Wan Verminator, try Perl and never look back, my friend.

You can use it for so many things, and add the arches as bonus! Just perfect!