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Well, other than just simply washing it, I know that there are products for hot rodders that have the satin and "dull look" paint to use to keep their paint jobs looking decent and uniform. You might check into that. In reality, I would probably paint it myself...perhaps with a semi gloss black (like the satin hood finishes) if you like the contrast with your top.
what others have said. Why didn't you get it clear coated or painted before installing it? Did you want the flat black look? I'm only going on the fact that you said black primer?
Installed a rear spoiler that was done in black primer.What can i use to keep it clean and not hurt the black primer finish.
What vendor did you purchase it from. Some are ABS and some are some are fiberglass. I have one for EOS and mine is the flat black. I happen to use Adams prods and he has a few different products that work well on in. I use his CS3 spray, which is a type of detail spray. I use it on the rear diffuser, and rear mug guards also.
Denim or dull look paint.
That is a paint and not porous like primer.
The point being you do not wish to rub the dull paint finish so that it begins to get shiny spots.
OP states the spoiler is black primer.
A common household product for denim paint is - Windex