You should use a roller if you need to cover walls and other wide spaces with a smooth, uniform coat of paint. Choose the best roller for the job based on the texture of the surface you're working with, and the sheen of the finish you're applying. There are basically just two features to consider when choosing a paint roller: nap material and length, but you should also consider the quality of the roller you buy. Don't skimp on price if shine and final appearance are important to you. For example, you should always choose a top quality roller if you're using any kind of shiny paint on smooth surfaces like kitchen cabinets or wood wainscoting. If you don't, you will have shedding of the nap fibers that can leave very visible "hairs" in the paint finish.
To choose your roller nap considering the condition of the surface you're working with. The surface texture, more than anything else, will dictate the length of nap you'll need to get consistent coverage. For example, a rough surface like thick drywall texture will require a long nap of 3/4 inch or more, while a smooth surface like wood or wallboard will be best treated using a medium to short nap of 1/2 or 3/8 inch.
After the surface condition, the coating type and finish will dictate the length of roller nap to use, and in some cases, the material to choose as well. For instance, when using a varnish, a high-gloss paint or any oil-based coating, a short poly/wool blend nap of about 1/4 inch is the best choice, particularly if you're applying these finishes to a very smooth surface. Check the quick reference chart below for more help choosing a roller by surface and finish.
The nap, or absorbent fiber of a roller is available in lengths of 1/4, 3/8, 1/2, 3/4 and 1 inch. As a general rule use a short nap for shiny and oil-based coatings, a medium nap for flat and eggshell paints and a long nap for any coating on rough surfaces like drywall texturing and brick or when painting hard to cover materials like bare masonry. A long-napped roller will create a good deal of spatter as it rolls back and forth so it should be reserved for situations like these where the paint must be forced onto surfaces that resist coverage, like concrete and brick.
Choose a medium nap of 3/8 to 1/2 inch for most paint jobs around the house including walls and ceilings, etc. These will produce a consistently smooth coat with minimal spatter and roller marks, even when using an eggshell or semi-gloss finish. Choose a short nap of 1/4 to 3/8 when you have a very smooth surface to paint with an oil-based or shiny finish. This would include applying high-gloss paint to drywall or plaster and when painting smooth wood like kitchen cabinets and built-in bookcases.
Roller nap is made using wool, mohair, nylon, polyester and poly/wool blends, of these, polyester is easily the most widely used for making both high and low quality roller nap. Natural wool, called lambskin, has been used for sometime to make high quality rollers for oil paint and varnish because of its low-shed tendency, but the trend is now toward a blend of polyester and wool for these coatings.
In addition to these traditional materials, high density and polyurethane foams are now being used to make roller nap and pad-type brushes. These materials can be difficult to work with when trying to achieve a uniform coat, requiring repeated strokes that can spread the paint too thin and leave streaks in the finish. Using these rollers can also get expensive as they can only be used once where a traditional roller nap can be washed out repeatedly using a roller spinner and used again.
Roller covers or sleeves are made to fit onto handles or frames, which can then be threaded onto an extension pole for a longer reach. Typical sleeves you will find at paint and home stores include 9, 7 and 4 inch sizes made to fit onto a wire frame. In addition to these sizes, mini, or sausage rollers are available for small and hard to reach spaces such as built-in shelves and kitchen cabinets.
The four inch size is just a stubby version of the larger types. A sausage roller is 1 inch round and about 6 inches long and made to fit onto a long, thin handle. This slender design makes it a better choice than the 4 inch size for easy maneuvering in tight places like inside cabinets.
Choose your roller size based on the size of the area you need to paint, a standard 9 inch tool is a good choice for painting interior room surfaces like walls and ceilings. A 7 inch size is also commonly available and may be a good choice if you're not comfortable with the slightly larger one, but as a separate tool it doesn't offer any particular advantages over the nine inch. You can also find a much larger, "jumbo" roller at most paint stores, but at 18 inches long these are way too big for most home diy. Jumbo rollers are mostly used by professionals, on large jobs with lots of space to cover quickly.
Always choose a good quality roller for your project, especially when painting inside the house. The higher quality will cost just a little more than the cheaper version but will last much longer making it more economical in the long run. You can usually rely on the price to correspond to quality when choosing a roller, but not always so look for these few telltale signs.
First, the nap on a cheap roller is not as dense as on a top quality tool. To test nap density wrap you hand around it, you should not be able to feel the inner core on the quality roller when you squeeze it in your hand. Even if it's a short nap, it will still have plenty of cushion between your hand and the core. Hold two rollers of differing price in your hands at the same time to feel the difference in nap thickness, the cheaper nap will seem to barely exist while the quality fibers will feel substantial and springy.
Another test of quality is the core itself. A cheap one will be made of cardboard or hard polymer-bond plastic, while a quality one will be made of phenolic resin. The materials used to make the roller will usually be listed on the label.
It's well worth buying top quality rather than trying to save a little money on the cheaper tool. The better one will stand up to many paint jobs and can be washed repeatedly, while the cardboard core in a cheaper roller will breakdown after only a couple uses. Even with a more durable plastic core, the nap on cheap rollers will shed excessively leaving unsightly fibers in your paint finish that can ruin the job.