In assessing negligence, what duty of care is owed by professionals?

Hypo 1: Sam asks Bob, as a favor, to paint his house. Bob has never painted a house before. Bob goes to the store and buys some paint called "house paint," then paints Sam's house. The paint Bob used is not meant for the type of wood Sam's house is made out of and, as a result, his house begins to rot. Result: The standard of care Bob owed Sam was of a reasonably prudent person, which arguably met by purchasing paint that was called "house paint." It shouldn't be expected that a reasonably prudent person would possess special, technical knowledge about what paint works with the wood on Sam's house.

Hypo 2: Same hires Amy, a professional house painter, to paint his house. Amy goes to the store and buys the normal paint she uses, then paints Sam's house. The paint Amy used is not meant for the unique, special type of wood Sam's house is made out of and, as a result, his house begins to rot. Result: Amy is a professional house painter, and so she is expected to have a higher degree of knowledge and owes a duty of care equal to her peers. Imagine if Sam called another local house painter to assess the damage, and that house painter pointed out the obvious mistake Amy made in selecting her paint. That would help establish a breach in Amy's duty of care. However, what if no local house painters were aware of how to treat Sam's exotic wood? Sam may try to show that in Zimbabwe, where the wood comes from, everyone house painter knows not to use the paint Amy used—but that wouldn't matter. What matters is whether Amy's standard of care is up to par with professionals in her community. The law doesn't care that Amy isn't omnipotent, only that she provides a service with quality expected of her professional alternatives.