One guideline is that if the first coat has sat for awhile,(weeks, months, years?), it's a good idea to lightly sand and re-primer dependent on primer type and the project concerned.

Thanks, I knew that one but some people prime twice for cabinets and stuff like that as well. Maybe just on wood.?.

Priming twice can be used to build or fill in barely detectable woodgrain.

Other than that........it's a waste of primer

It can be helpful on new woodwork, esp. that to receive an oil enamel finish. One coat of primer doesn't do justice to end grain, small checks and other defects. If you follow the recommended prime/ then caulk order of operations, any areas that needed resurfacing with spackle will need an additional coat of primer of those things can shine through,
And the there's MDF. It needs a sealer and a primer, I don't know of a single-step product that can do as well.
Casey

Thanks guys!!!

Priming twice can also be helpful if you have something in a really really bright or dark color. For example, the only way to cover the yellowish sandish color all our house was painted in on the inside, with other colors like blue, was to either use several coats or to prime the wall twice.

Priming twice can also be helpful if you have something in a really really bright or dark color. For example, the only way to cover the yellowish sandish color all our house was painted in on the inside, with other colors like blue, was to either use several coats or to prime the wall twice.

Not so if using a quality primer. One coat of primer is all you need,it will not complety hide what you are priming but that is not what it is made to do, the finish coats will do the hiding.