By Relocation.com
For doors that get a lot of use, use a semi gloss or hi gloss alkyd based paint. You can also use latex enamels as these also hold up well. Before repainting a door previously painted with a high-gloss paint, use a de-glosser to dull the finish. For bare wood use a primer.
It is best to remove the door and lay it flat on a sawhorse before painting.
Be sure to paint the bottom and top of the door so as to prevent sentry of moisture that can cause the door to swell or warp. You can use a mini pas paint applicator that lets you paint the bottom edge without removing the door. You can use a roller, brush, pad or sprayer to paint a door. Plan to back brush with a brush or pad if you use a roller to prevent a stippled finish. If the door is paneled, sue a brush and paint the panels first, then the rails or cross pieces and then the vertical stiles. Always paint with the grain and brush out form a corner rather than toward it.