Primer - Paint - Why don't they just put primer in paint?

I know one must use primer on raw wood for good performance of the paint that ultimately covers it. My question is - what is in primer that does the "priming" ? It looks for all the world like regular old paint, so why don't they just put the stuff that's in there right in the paint so we can just paint without multiple steps?
Yeah, I know that to pro painters this is probably a dumb question, but I'll bet there are others out there who wonder about that every once in a while too - so flame if you must, but I'm just wondering.
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I've bought just such a thing at Walmart. McCloskies (spelling?) brand. Used it on some block piers at a cottage. Been there over a year, no problems yet. Block did suck it up a bit but I did one coat and it looks fine.

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Steve Henderson wrote:

Same reason they don't put the peanut butter in the jelly?
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Norminn wrote:

They tried, a few years ago. It didn't sell!
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Norminn wrote:

http://www.smuckers.com/fg/pds/default.asp?groupid=2&catid=4&prodid 6
Brian
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On 13 Sep 2004, Steve Henderson wrote

Primer, as far as I can tell, sticks to the wood but doesn't sink in.
Maybe it's not possible in chemical terms to marry a true "non-sink-in" characteristic with a regular paint -- which is either absorbed by the underlying material or has to sit on top of another barrier paint layer.
(??Maybe??)
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one quality of primer is 'teeth'. little 'hooks' in it that grab.
randy

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On Mon, 13 Sep 2004 14:25:06 -0500, Steve Henderson wrote:

You want different things in a topcoat, such as depth in a high-gloss. Primer you're just hoping is a very sticky base that never peels and sticks to anything. The depth (thus, low solids content) is opposite what a primer has (high-solids content to hide the underlying surface and provide a starting point for your color). That's at least two reasons for separate base/top.
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Except when you don't need gloss, and high solids means good coverage and hiding.
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Depending on what you're trying to cover, basically paint is primer. Some block stains, some fill in rough surfaces, etc.
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Steve Henderson wrote:

Paint is a slurry combining "tinting agents", "binding agents", and something to hold it all together.
Primer is heavy on the binding agents. The binding agents can cause the paint, when used as topcoat, to clump against itself and create a lumpy appearance when viewed up close. This can be just fine in a lot of cases, but most people want a better finish than primer can provide.
Put it this way. Primer holds on tight to the wood, and to the topcoat. But without primer, the paint holds on tight to itself. Thus, peeling and coming off in multi-inch sheets.
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Steve Henderson wrote:

Primer - oil primer - has a much higher percentage of linseed oil. It's the oil that does the priming.
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dadiOH wrote:

have a better understanding now, and won't resent (well, as much anyway) buying separate products in the future <grin>. Steve
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If it makes you feel better, just consider it to be a second coat of paint and ignore the fact that the label says "primer". The problem that I'm having with it is that I need Alkyd primer and I seriously HATE cleaning up oil paint.
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A MAJOR help in my opinion is to preload the brush with the proper solvent, thinner for oil paints. Stops the finish from starting to dry near the ferrule stiffening the brush and makes clean-up much easier. I grew up with oil paints and learned this hint a few years ago. Wish it had been 60 years ago!

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Yes, and the same goes for water based paints (in that case preload the brush with the proper solvent again - water.)
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