Physan 20 is a mild bactericide/fungicide that is best used to help prevent problems rather than used to treat problems.
I use it on orchids that are kept moist most of the time, to prevent fungus problems in the medium. For me that's phalaenopsis which are also susceptible to bacteria problems. I haven't tried it on paphs so I can't comment on them. Since my cattleyas, encyclias and vandas dry out completely between waterings, I don't worry about fungus in the medium although it certainly can happen.
I use Physan 20 at 1 teaspoon per gallon to soak the medium of all my phalaenopsis, approximately once a month. I don't really spray the leaves but it does get on the leaves and it doesn't hurt them. I don't rinse it out of the medium after applying because it's very mild and takes a while to work. Occasionally during the summer when it's very humid I'll spray all my orchids and even the orchid benches and the floor of my orchid growing area to help prevent fungus problems.