My opinion has always been that is the responsibility of the painting contractor to educate the customer on what is good paint, not the other way around. I don't know a single painting contractor that shops at Walmart. If you told him to use Duron and he bought Walmart paint, that's a problem that should have been addressed before the job started. I know you probably didn't expect to have to babysit the guy. My guess is that his bid included buying cheap paint from Walmart and the Duron paint costs more.

It sounds like the actual problem with your walls that he painted is just a matter of coverage. This doesn't mean that he skimped on the priming step. Coverage comes from doing multiple coats and if it's not covered, that means he skipped a coat (whether it's the primer coat or the finish doesn't really matter). I prefer to prime once and then apply two finish coats. If the finish is flat, then one coat of finish is usually enough. It is certainly an idustry standard that when a pro painter does a job, you should end up with complete color coverage. If it is not covered, then you should be able to point this out to the contractor and he should finish the job for free....assuming he is a reasonable person of course and wants to keep his rep up.

As for how you have the conversation, I'll tell you how it should be and how it is for me. There should be an end of the job walk through before you give him payment. There should also be a contract that spells out what is included. If there is no contract, then you need to discuss what is reasonable and what is industry standard. The best way for you to handle this is to act like there is a walk through...act like you expected one because every contractor you have used in the past has had one. A lot of people actually use blue tape to mark the areas of concern. If he is a real painter that walks away from jobs that are not completely covered, then I'm sure he is used to the blue tape routine.