New Wood:
Start by cleaning to remove sawdust dirt and other contaminates.
Prime with a quality latex or oil base primer. When dry fill all holes with spackling compound and caulk all cracks.
Lightly sand with 120 grit sandpaper and wipe clean using a damp cloth or tack rag prior to applying your finish coat of enamel.
Two coats of finish paint are recommended with sanding in-between using 150 grit sandpaper.
Previously Painted Wood:
It is never a good idea to apply latex base paints over oil base. You can determine what you have by rubbing the surface with a rag and denatured alcohol. Latex base paint will soften and leave a residue on the rag,oil base will not.
If your existing coating is a latex simply patch holes and dings, sand lightly using 120 grit sandpaper, caulk cracks,wipe clean with a damp cloth and apply your finish coat of enamel.
If in fact you have oil base you can prepare and recoat using an oil base enamel or if you wish to switch to a latex paint system it can be done but preparation is critical.
Converting to a latex base paint is something that is going to have to be done eventually as most all oil base paints are being phased out by manufacturers in order to comply with new EPA standards. A thorough deglossing of the surface is necessary. Use 120 grit sandpaper and sand all surfaces. "Liquid Sandpapers" are not recommended as they leave an oily film. Clean well with fresh water and rags,dry and apply a coat of latex primer designed for adhesion. We use and recommend Zinsser 1-2-3, this product has tremendous adhesion properties. Allow to dry completely,at least 48 hours,sand lightly using 120 grit sandpaper and apply your finish latex enamel.
Pro Tip:
When painting wood sash windows or panel doors, always paint the innermost sections first and finish off with the outside sections. Example, a six panel door. Start by painting the panels as the brush will overlap some onto the stiles. When the panels are completed then following the direction of the grain of the wood, horizontal stiles first, vertical last, paint the stiles and lay off when completely covered in one long brush stroke. This technique eliminates a lot of excessive brush strokes.
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