Hello Phil

% of acid bath depend on the paper and the process. Some paper just need to be brush with a 1% acid oxalic solution and other may need an 5% bath for 5 minutes. This is link to he buffer in the paper.

On some paper this bath could play on the sizing . But this depend on the quality of the paper. I don't think it will happen on arches watercolor hot press which have a quite heavy gelatin sizing.

Another thing I think about is that normally you got to use a free acid oxalic ferric oxalte solution , for you kallitype . If you did so, the acid you put inyour paper perhaps act with your sensitizer. I am not a kallitype user so I am not sure of the effect of some acid oxalic in the sensitizer.