Most people are under the assumption that painting will cover up defects in a substrate being coated. Although part of this assumption is true, most of it is not. An opaque finish will actually show more irregularities in wood than a natural, clear finish does. This is especially a concern when it comes to joining two pieces of wood together. No matter how good of a joint you achieve through careful fitting and sanding, the seam will typically be revealed once the paint dries. It's one thing if you have pre-designed this detail into the work, but if you want the painted finish to be seamless, you must prepare the woodwork differently. Consider the painted finish on an automobile. Brand new, the car's body has a continuous sleek look as your eyes roll over each contour and crevice. When the body gets dented or scratched, the defect shows up instantly because the line of sight has been broken. The only way to remedy this visual break is to have the surface repainted. Wood millwork acts similarly. The same attention must be given to what you are painting as how you are painting.
A few important things to remember when specifying painted millwork:
1. First, the higher the sheen, the more important the prep is before painting. Light reflects more as the sheen gets higher, which means defects will show up easier.
2. Next, when painting everything the same color, consider the fact that all the surfaces will become homogenous. There will be no mis-direction of grain to deceive the eye, which means you will easily discover poorly prepared work. Also, the color on intersecting planes will reflect light differently, creating slightly different colors in the shadows.
3. And finally, select the joinery that helps you best achieve your goals. The accompanying slide show offers a few ideas on how to minimize visual distractions when preparing casework for paint.