Thanks for the replies everyone

The paper is Arches WC color paper which I get from a local Art Blick store:

The paper is well senstized and allowed to "set" for a few minutes before using a hair dryer set on the low cool setting on the back side until the paper is "bone dry." Tween is not used. The first 5 minute wash is in distilled water with a pinch of citric acid. The second wash (2 minute) is in tap water with a pinch of cirtic, and the 3d, also 2 minutes, is in running tap water. The local municipal water source is snow melt from Hetch Hetchy (Yosemite NP) and using distilled water is probably not needed in my case but I use it for the 1st wash anyway. After washing the paper is hung to dry in a dark closet.

James says that Arches is "fair" for VD. I wonder if the mottling is a result of the citric acid?