I never use one coat. You will find that the first coat does not go on
thickly without running, it's main job is to bond the the layer below. If
that layer is old and very hard, the paint will not give a lot of coverage
as it slides around on the old coat.

24 hours later, a second coat will lay thick, quickly laying onto and
bonding with the "not fully hardened first coat" giving a thick, full
coverage layer with no showthrough.

"Doug Miller" wrote in message
. com...
In article , Minnie Bannister
wrote:
Applied a coat of Sherwin-Williams ProClassic Extra White to a
pre-primed door yesterday (primer has a very, very pale greenish tint)
and found that there are still dark patches showing through.

Shouldn't one coat of a decent paint have been sufficient?

If the can claims one-coat coverage, then, yes. But I'll bet the
directions
say to use two coats. And you just discovered why. :-)