So I recently tried to remove some sticky sap from my Thule rooftop cargo bin, just soap and a sponge the same way I was also scrub it off car -- but after the water dried I found a bunch of faded/discolored patches on the surface. Presumably the sponge I used had slightly roughened the surface and I think the discoloration is from lightly scratching the shiny finish on the bin. (NOTE there are two types of surfaces on Thule bins, either a ultra smooth and glossy style, or a textured but still smooth and polished look, which I have the latter, its a Thule Force model.)
I've contacted Thule and the rep suggested I just spray paint the whole surface, which I do not think would work well. I'm wondering if anyone here has suggestions for spot-polishing and maybe applying some kind of shiny clear-coat onto the bin. This might even be a question for a car forum since I imagine whatever clear coats you put on a car paint would work well here: covering a plastic shell that needs to withstand the outdoor environment.
I'd try fiberglass polish for boats. I like Starbrite Premium Marine Polish. I have used it on my sail boat but not tried it on my glossy silver Thule box however. Possibly you can use an electric buffer if it is surface scratches.
However, I don't know if glossy Thule boxes are painted or if the color is throughout the resin so that may make a difference. If they are painted with a coating then if the coating is damaged you may needed sand it down and repaint, at least with clear coat. Polishing will not restore damaged clear coat although it may slow further damage.
Thanks for the recommendation ! I'll take a look into that and maybe apply it to a spot.
Also I replied to the Thule support asking about the material of the bin. They say "there is no coating, it is a layer of plastic with the paint color baked right onto the cargo carrier.
The plastic is made out of ABS.."
A little unclear but I'm reading it as a unibody of black ABS plastic, and a black paint that cures with a glossy finish was applied on top.