While painting your walls, the right roller covers make a huge difference in finish quality. To consider an extreme example, painting a bedroom wall with a 1-inch roller cover -- one of the thickest you can get -- would give you sloppy, drippy, bumpy walls, hardly the pleasant look you want for your sleep chambers. The following list progresses from bumpy textures to mid-range to smooth, along with recommended roller covers:
Purdy 1-Inch Nap Roller Cover
Best for: Indoors on extremely bumpy, textured walls and ceilings. Outdoors on masonry or highly textured wood.
You Need: 1-Inch thick roller cover.
Details: One-inch nap paint roller covers are those big, fluffy monsters that are used mainly for very uneven and bumpy surfaces. Unless that describes your walls, return them to the store immediately.
Thick-nap paint roller covers are a nuisance unless you're dealing with a surface that really needs it (usually painting ceilings, and more specifically that cottage cheese or popcorn style surface). So, stay away from them if at all possible. These paint roller covers will suck up all of your paint and give you sloppy paint coverage.
Wooster 1/2 Inch Nap Roller Cover
Best for: General indoors wall painting on relatively smooth surfaces.
You Need: 3/8-inch or 1/2-inch thick roller covers.
Details: Your standard-issue default roller cover. It's hard to go wrong purchasing this one. Three-eighths inch (3/8") and half-inch (1/2") paint roller covers are found in abundance: you can buy in bulk and save. But be careful of cheap roller covers with cardboard spindles.
They may give you more trouble than you would like.
Wooster 1/4-Inch Nap Roller Cover
Best for: Specialty work on very smooth surfaces.
You Need: 1/4-Inch thick roller covers.
Details: A very flat, hardcover. It lays down the smoothest paint of all covers listed here, but few materials are appropriate for it. Quarter-inch roller covers feel almost like they are covered in velvet -- very flat and hard. Most walls have too much texture to be adequately covered by the 1/4" roller covers. Only the smoothest of smooth materials (Masonite, for example) will accept paint from 1/4" roller covers.