BOTH types of 5a-reductase convert testosterone into DHT.
As I've pointed out numerous times, studies with topical finasteride have produced results that are all over the map. I can show you a well-designed one that found a very slightly REDUCED hair-count from topical finasteride on the scalp, although I imagine that was probably just a statistical fluke. But personally, I'd recommend against using it, until the experts can explain why it seemed to work a little bit in some studies, and not at all in others.