I've had mixed results with Arches paper. I too would get some blotches and believe it's caused by the sizing. The blotches would occur after long wet times and would be present regardless if an acid presoak was done or not. I think some batches of Arches are not evenly/consistently sized and the blotches result. I've never had this problem with Arches Platine, but it would frequently happen with Aquarelle hot press.

Regards, Pete