Re: How to detail semi-flat / satin black paint ?
You will wash it like you would the rest of the car, do not polish the area and only use a chemical cleaner (no abrasive). If on a budget use something like Opti-Seal to protect it, if money is no object get your hands on SV Opaque was and might as well the get Opaque cleaner which is made for matte finishing. You could also get Insignis which is good wax for matte finishes.
Like I said SV is the expensive option but Opti-Seal will not alter the looks of the matte and give good protection and what I used before i got Insignis.