Leave doors hanging on their hinges while you paint them so that you can paint both sides at the same time. You can remove most modern lock sets in less than a minute (and replace them in under two), so removing them for painting is easier than masking. Make sure that you do one or the other.
You must seal all surfaces of new doors to prevent moisture from entering the door and causing it to warp. This step is critical for a solid-wood door or a solid-core veneered door. You can even seal the surfaces of hollow-core doors (the most common lightweight interior door), which aren't as prone to warping. If you don't plan to take the door off, slip a mirror under it to see if the bottom edge has been painted. If not, either use a mini-painting pad that enables you to paint the bottom edge or remove the door from its hinges, apply a sealer to the bottom edge, and rehang the door to paint the rest of it.
Rolling a flush (flat) door
Paint a flat door with a brush, pad, or roller (with a 1/4-inch nap or foam sleeve). If you use a roller, backbrush immediately with a wide brush or pad to smooth the roller stipple. The texture that rollers impart works well for walls and ceilings but isn't attractive on doors or wood trim.Doors with a luan mahogany veneer have a rougher texture than those with a birch veneer. Although you'll never get a mahogany door as smooth as you can a birch door, sanding between coats helps, especially if you're priming and painting a new door. After painting the door itself, paint the doorjamb and casing, beginning at the doorstop and working out.
Painting raised-panel doors
Paint a raised-panel door with a brush and paint with the natural grain of the wood. Painting around the panels is time-consuming, making it difficult to keep a wet edge.
One commonly recommended approach is to paint all the panels first. But this approach works only if you cut in carefully and avoid getting paint on the faces of the stiles, muntins, and rails that frame the panels. If you get paint on a stile while painting the first couple of panels, for example, the paint may set up before you're ready to paint the rest of the stile.
For most painters, the best approach is to paint the top pair of panels and the stiles, muntins, and rails around them. Then move to the next lower pair of panels, and so on. Using a paint conditioner keeps a wet edge longer, makes the paint go on more easily, and improves its bond, which are all important qualities for door painting.