Depends on the shoe and the amount. I mostly wear sneakers and never had any build up. You just put a bit of powder and shake it in the shoe. Humidity will diss

Depends on the shoe and the amount. I mostly wear sneakers and never had any build up. You just put a bit of powder and shake it in the shoe. Humidity will dissolve the powder, some will wick into socks, protecting them as well. If you still see some in the shoe then no need to add more. Best is to try it and make your own experience. Boric acid is quite cheap.