David Nebenzahl wrote:
This is a serious question, not a troll or a trick, as I really don't
know the answer to this: what's the difference between primer and
"regular" paint?

First of all, some ground rules: I'm hoping to get answers from
someone who actually knows what they're talking about (any paint
chemists out there? or at least someone familiar with the inside of
the paint industry), rather than the usual speculation. Therefore,
any answers from "hallerb" or "ransley" will be politely ignored.

If someone forced me to explain the difference, my answer would be
something like, "Well, duuuh, I think primer has more solids or
something; or it's made to stick better. Yeah, that's the ticket--it's
stickier!" Obviously, no definitive understanding of really what
primer is and why we're 'spozed to use it.

The primary purpose is to provide a surface with uniform absorption
characteristics. It does that because it *does* have more solids (that is
also why it is heavier.) I haven't checked to see what the solids are
nowadays, used to be stuff like whiting (powdered limestone). The solids
also help provide a smoother surface by filling in grain and minor surface
scratches/dings.

With luck, the primer will also be easy to sand so you can do so and get a
baby-butt smooth surface. Most aren't.
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Extra credit for answers to the following:

o Actual differences between oil-based primer (which I intuitively
consider to be better without really understanding why) and
water-based (latex, acrylic, etc.) primer.

Well, duh. Oil based primer has oil in it

Actually, it has a higher proportion of oil (compared to "paint"). That
means it is relatively soft when dry and that means it resists chipping or
loss of adhesion. That is also why it doesn't sand well...for a prrimer to
sand easily it needs a high proportion of solids and less oil. Get one
thing, lose the other.
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o Are there any paints made today which can be considered
"self-priming" and which do not require primer?

Any paint. It will just require many more coats to get to the point that
one coat of primer will give.
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The assumption is that we're talking about painting wood products
here, not speciality paints like for metal or other materials.

Which is why I didn't bother talking about latex.

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dadiOH