Normal to use side of thumb while practising arpeggios?!?

[kujiraya]

When I'm practising arpeggios, when my left hand's 4th finger goes over my thumb, or when my thumb goes under the 4th finger, I am hitting the note using the side of my thumb, where the nail joins with the skin. After an hour or so of arpeggios, this spot gets sore and even a bit squishy! I can imagine a callus forming there in a month or two. Am I doing something wrong with my arpeggio technique? Or are arpeggio thumb calluses just par for the course?

[faulty_damper]

I don't think any decent pianist has any callus on their hands.  Arm shift; use the arm more.

[outin]

I have the same problem, although not with arpeggios. That part of my thumb gets sore sometimes when I play for a long time with my piano because the edge of the key is not completely smooth and I have to use that part of the thumb to reach octaves and chords. My skin on that part seems to be especially thin and sensitive, I would have expected it to harden already...Never have this problem with my teacher's pianos, her keys are smooth and much easier to play anyway.

When it bothers me too much I put a plaster on my thumb 

[andreslr6]

I don't think any decent pianist has any callus on their hands.

I must be a very undecent pianist then.

I don't think anything is wrong as long as it doesn't hurt you and as long it solves the music. What would you do when you have to play 2 notes with the thumb? you have no choice there, or glissandos? a lot of people get callus by practicing/playing glissandos. I have a callus on both pinkies, thanks to Prokofiev, and without them I wouldn't be able to have a good grip on those leaps. They'll naturally form, don't be afraid of them, just as string players get theirs, and as my piano teacher has, and my teacher's teacher too who is in fact more than just a decent pianist

I'm assuming there's no tension when you do what you say you do, of course, if there's tension then that has to be corrected.