Is charging by the hour a common way of bidding wall repair?

[Marian Smith]

I asked a painter who lives across the street from my mother for a bid on repairing some drywall cracks...spackle, mud and tape, whatever and hand trowel texture. I stuck blue tape on all the cracks so he could see the scope of the job. He walked through and said $35 an hour. I asked for a ballpark on the job and he said he really couldn’t tell until he got workng. I am not going to use him because I want a firm bid, but is this common or accepted? I pay plumbers by the hour, and hvac and electricians. But I cant imagine what could come up on what I consider a paint job that he couldnt anticipate. Thanks.