Your comment made me head to the internet, because I realized I didn’t know that either. This is lifted from a podiatrist’s website, so I am assuming it’s got some scientific basis.
I will say though that I have seen some pretty ugly blisters under calluses, thankfully, not on my feet!
Tip 2. Toughen Your Feet to Prevent Blisters
They don’t call a newbie a tenderfoot for nothing! Your soft, pink feet will have fewer problems with blisters if your skin gets a little tougher.
Calluses Are Your friends: As your feet get more of a workout, they build up calluses. These are your friends—you want calluses, which act as a natural pad against the friction that forms blisters. Do not give in to beauty and shave off or pumice down the calluses, at least not until after the long walk.
Tannic Acid to Toughen: Marathoners and long-distance walkers may want to toughen the feet with 10% tannic acid or a tea soak. Apply the tannic acid to your feet, or soak in strong tea, twice daily for two to three weeks.
Moisturize Away Heel Cracks: To prevent your calluses from drying out too much and developing painful cracks, moisturize your feet after each bath or shower with a good foot cream or hand cream.
So,
@Dani7 are you surprised that someone would be using it to “toughen” his feet? Seems like it’s not indicated for that purpose!