Perhaps you should post this in the Houseplant Forum, or in the Plant Clinic Forum. I'd be happy to help you, but I am simply not experienced with PM on staghorn.

thanks.

There have been numerous postings of success treating powdery mildew with milk. Mix up a spray of 3 tablespoons milk (any kind) to one cup of water and spray the plant throughly. I haven't tried this so would be interested in hearing if you have good results.

Also, rather than repotting your fern do the following. Using a large serrated knife cut a deep circular core out of the center of the fern. Fill the remaining hole with fresh potting soil. This method works well on potted ferns including Rabbit's foot and Boston ferns. It may be necessary to do this twice a year.