I am trying to better my slap style, but every time I slap for more than 20 minutes, I get painful blisters on my index finger. I switched to middle also and got a blister there too. Is my technique wrong or is it kind of like building up calluses? Either way it is extremely painful.
It should build up calluses if you keep playing at all.
I'm not a bass player but I'm pretty sure its calluses. when i started playing guitar, the fingers on my left hand would sometimes get blisters if i play for a long time. these eventually just toughened up. so just push through the pain and eventually everything will be alright
Don't push through the pain. That can help in literally no way. When it gets to the point where you're in actual pain (as opposed to discomfort) just stop playing for a bit. You might even find you can pick it up again in a short while. Pushing through the pain, however, is the most ridiculous sentiment - the pain is there for a reason.
when I learned to slp I got HUGE blisters on my index and middle fingers. I drained them (not sure i that was a good idea) and let them heal, worked on guitar playing for a couple of days with a pick, then after a while started playing again and I haven't gotten blisters since.