From an interview with Steve Harris of Iron Maiden:
Do you get much damage to the fingers themselves?
Yeah, the first couple of weeks I get blisters. Then they burst. They're really, really sore for a few days, then they go really hard, and that's it. They stay like that for the rest of the tour. The only bugbear is that when you play somewhere really, really hot and get really sweaty, your hands go soft, and that completely changes your sound. Your fingers tend to sort of sink into the strings, almost. And there's nothing you can do about that, it's the temperature of the place.
That's why I prefer to play in places that are a little bit colder. Also, that maintains your treble output, and the attack is more even, the fingers stay hard and solid. Very big difference: It's like someone put a muffler over your speakers. So, you just have to add more top-end to try and compensate, but there's not a lot you can do.
With the blisters, have you tried putting stuff on to harden them?
Yeah [
dismissively], tried that. Doesn't work. I tried that Nu Skin stuff, when I've had a blister that's come open, when you're through to the skin underneath, that's really sore. But once you get out playing, get the adrenaline going, you tend to be alright anyway. It hurts a bit, but sometimes the pain—you just grit your teeth and get on with it. Weird.
Source:
Interview: Steve Harris on Iron Maiden's Foundation | Bacon's Archive