Roller marks, also known as poor flow or leveling, can be caused by a number of factors. While they are more common with low sheen paint like flat or matte, they can occur with semi-gloss paint, too. Once they occur, the only solution is to repaint, fixing what caused the problem previously. The marks may be due to your paint, equipment, painting technique or the conditions in which you are painting. For that reason, you may need to start fresh with new equipment and paint before you attempt to repaint the surface. Once you've selected the right tools, painting correctly will help you achieve a smooth coat the next time around.
Choose the right roller cover material for the paint. For latex or water-based paints, use a cover made from synthetic materials like nylon, Dacron or polyester. For oil-based paints, use natural covers like mohair or sheepskin. Blended covers made of polyester and wool can be used with either type of paint.
Select the right nap thickness for the surface. For smooth surfaces like drywall or plaster, use a roller with a nap between 1/4 to 3/8 inches thick or one marked smooth or semi-smooth. Semi-rough surfaces like light stucco or rough wood need a nap that is between 3/8 to 1/2 inch thick or that is marked semi-rough. More textured surfaces like concrete block or brick need a nap that is 3/4 to 1 1/4 inches thick or that is marked rough to very rough.
Purchase high quality roller covers because they tend to be more uniform and consistent. Also purchase a high-quality roller frame. Not only will a better frame make it less taxing on you to paint, it will provide a smoother coat of paint.
Purchase high quality semi-gloss paint. Cheap paint doesn't spread as easily as more expensive paint. Make sure you have enough paint for the job. Trying to spread paint too thin over an area often leads to roller marks.
Add a paint conditioner to the paint. Paint conditioners make the paint smoother and keeps the edge wetter longer, which helps you avoid lap marks.
Sand the surface until smooth with 220-grit sandpaper once the paint with the roller marks has dried thoroughly. Wipe away any dust with a tack cloth.
Wait to paint until the surface is in the shade and cool to the touch. Painting a hot surface or one that is in the sun will cause the paint to dry too quickly and can lead to roller marks.
Cut in the edges of walls, trim, ceilings and any fixtures before painting. If using oil-based paint, only cut in one section at a time to ensure it doesn't dry too much before going over it with your roller.
Pre-dampen your roller with water if using latex or water-based paint. Shake off any excess water before loading the roller with paint.
Load the roller with paint so it is full but not dripping with paint. For large jobs, use a 5-gallon bucket with a roller grid to load the roller instead of a pan.
Break the surface mentally into 3 x 3 foot sections to paint, if necessary. Breaking a wall or ceiling into smaller sections helps you avoid "working" the paint by applying it too thickly. The paint can should list the "appropriate film build" or how thickly the paint should be applied.
Start in one corner and apply the paint over one 3-foot section. Roll the paint using M and W shaped motions until the area is covered with paint. Roll slowly starting in an upward motion to avoid splatters. Don't lift your roller from the surface until you are finished. Roll as close to the edges and corners as you can. Finish up the area by rolling the now unloaded roller in a straight line up or down on the area.
Move onto the next 3-foot section. Your goal is to get to the next sections before the edges become too dry, which can cause the paint to become uneven (known as lapping).
Let the finish dry according to the manufacturer's recommendations (listed as re-coat time), then, sand the surface lightly with 220-grit or higher sandpaper. Sanding will allow the second coat to grip better to the first one. Repeat the above steps to apply the second coat.