With Sherwin Williams has been excellent. However, what I would do is find out what Sherwin Williams store your painter is buying the paint from. Go to that store, and ask the Store Manager to either come out to your house for an evaluation of the situation, or ask him to have his district technical representative come over.
Get them involved from the start, and you will not only get good advice on what product to use, but they will also stand behind the job. From a material standpoint at least.