Sometimes when a car' primer coats are wet sanded the residue gets into the crevices. Problem is when car isn't prepped properly this residue can be left there and sealed in when the clear coat is applied. Honestly if you used a nylon bristle brush and prep solvent and it is still there it may be the above problem. Usally this happens in wheel well and door jamb areas due to water residue dripping during sanding process. Hopefully you just aren't cleaning hard enough and it is final compond residue.