For water based paint, use the best quality roller cover you can find, with synthetic nap. For oil based paint, try a mohair roller nap. 10 mm is good; pre-wet the cover before starting: water for latex paint, paint thinner for oil based paint, and then remove as much of the water or thinner as you can by squeezing then rolling on newspaper. If possible, take the doors off the hinges, and place horizontally for painting. You may need to thin the paint SLIGHTLY, before painting. Make sure there is plenty of paint on the roller. Don't apply any excess pressure to the roller while painting. Roll an area only 2 or 3 times... don't keep working it.