Proper Painter Roller and Paint

OK, someone at a large store advised me that if I was painting with flat paint on a smooth wall, I should use roller with a thicker nap. Why?
I see on the web that some sites advise using a short napped roller with glossy and semi-glossy paint. Wouldn't a thick nap provide greater chance of drip marks?
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I am not a pro painter.
The quality of your paint job after proper preparation is in the quality of your paint, roller, brushes and skill.
A 3/8" nap on almost any grade roller will yield good results with a flat interior paint on a smooth wall. The flat paints are formulated to be "self leveling" and you have to work at it to screw them up. The thicker nap allows you transfer more paint to the wall. This perpetuates the one coat myth. That said; I use a 3/8" roller for all interior walls and lightly textured ceilings.
It is harder to apply an even coat using semi or glossy paint. Minor imperfection is the coat level result in more or less gloss resulting in an uneven appearance.
If you are experiencing drip problems using a roller, you are putting it on way to thick.
Roller marks occur when rolling wet over dry but can be smoothed out if you do it as they occur.
As mentioned above, plan on 2 coats for anything you care to look at later.
Colbyt
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" someone at a large store advised me" That is an oxymoron.
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flat paint can be applied messy and still look good. more knap holds more paint, rolls on faster.
randy

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Wrong. Shorter the nap the smoother the finish. 3/8 is smooth walls. You want a really smooth finish, use one of the new foam rollers. No "orange peel" like you sometimes get with nap rollers. Larger nap rollers are for rough surfaces like cement block..to get into textured surfaces.

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