It doesn’t have to be dry, unless the actual Najasa is there. For example, if one’s hand touches urine, then his hand touches the doorknob and drops of urine are transferred to the doorknob, then in this case the Najasa itself is present on the doorknob. Urine is considered Ayn al-Najasa, so the “intermediary” discussion doesn’t apply to the doorknob when the Najasa is present. Yes, if the actual urine wasn’t present, then the discussion on intermediaries applies.