Most of the crud on the inside of your wheels is probably brake dust. I just stripped the clear coat off of my 88 wheels and inside was clear coated also. I would think an 04 wheel is completely clear coated to protect from road salt and the like. My 88's wheels were crazed and nasty looking, stripping them made them look good, I applied a coat of aluminum polish after I was done stripping, I don't us the car in the winter, without the clear coat the wheels would like crap in no time......