Thanks, Denny and Andrew. I tried both methods tonight. 1 drop of citric acid (I don't have any oxalic) per 1 mL of kallitype sensitizer didn't work for me. Soaking the paper in 3% citric acid this morning then hanging to dry all day gave an improvement, but still a little residual blotchiness - maybe I didn't soak it long enough. Using the sensitizer with added citric acid on the acidified paper worked great.
It looks like this paper will work for me, I won't have to donate it to my wife's watercolor efforts!