What final grit of sandpaper for paint-grade projects?

This is probably a really basic question. But is there any general rule of what the final grade of sandpaper should be for a project that you plan to prime and paint?
I have always sanded up to 220 but wonder whether that might actually be "too fine" for paint to stick optimally.
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For wood:
IMO, if it is an "art piece" like a jewelry box that won't get much wear or cleaning, then go to perfectly sanded 220.
If it is something like a shop built kitchen cabinet where you will put the normal one coat of primer, then two coats of finish, a meticulous sanding with 180 grit will work well. Use 220 to sand spackled and filled areas as they are prone to scratches.
Outside (as in house), the rougher the better as the paint will have to face endless expansion and contraction. It needs all the help it can get.
For metal:
You can't get the surfaces smooth enough for something like a metal door. I still don't go past 220, but since metal paints go on much thinner, you have to be even more careful when sanding as the imperfections of the surface will come right through metal paints. No amount of paint will hide the imperfections unless you are going to put enough on to compound out as much as possible. Still, proper sanding is the key.
For fiberglass:
See notes on metal.
Good sanding technique is (to me) even more important than than the grits, paper, and machines.
But I think you brought up a good topic. Most people really over sand.
Personally, I think it is because they are putting of finishing as long as possible. =^)
Robert
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On 2/14/10 2:24 AM, snipped-for-privacy@aol.com wrote:

Have you ever lightly sanded the primer or first coat of paint, before applying the final coat? I had a friend who painted houses for a living and the only things he would sand were those metal exterior doors... very lightly. The finished door was so smooth, it looked like it had been powder coated.
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I always sand to 150 and then prime it. Then I lightly sand the primer with 320 before I put the paint on it.
JP
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Maybe it's true, I read an article about too much sanding. 220 grit sounds right. I lightly sand the primer, dust off, then ready for a smooth finish. Sand again between coats using 320 grit.
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