Fiberglass doors don't warp, rot or rust and are fairly common in newer houses. Typically used as main-entry and other exterior doors, they usually come primed and ready to paint. The best paint to use depends on several factors: the manufacturer's recommendation, the condition of the door and the look you're trying to achieve. Some fiberglass doors can even be stained and varnished to look like wood.
Unless the manufacturer's directions recommend against using water-based paint, 100 percent acrylic paint is a good bet. Check the hinged edge of the door for a label; often finishing instructions are printed on a foil label there. The Rohm and Haas Paint Quality Institute recommends a "top-of-the-line" 100 percent acrylic paint in satin or semi-gloss finish. If you only need to paint one or two fiberglass doors, a quart of paint will be enough--so it makes little sense to try saving a couple of dollars on inferior paint.
Alkyd (oil-based) paint works well on fiberglass doors, although the long drying time makes it inconvenient for an entry door since you need to keep the door open slightly as it dries. Alkyd paint is also more likely to fade if exposed to a lot of sunlight, especially if it has red or blue pigments added. Ask paint store workers if the color you are considering is likely to fade or will remain colorfast for years. Like acrylic paint, alkyd paint will adhere very well to the fiberglass surface as long as it's clean and dry.
If your fiberglass door is older and worn, or if it has a very shiny, slick or varnished surface, it should be primed. Clean and sand the door first, and remove any loose finish. Look for a bonding primer and make sure it's specified for exterior use unless your fiberglass door is inside. Bonding primers come in several formulas. They can be oil-, shellac- or water-based. Oil-based primer tends to be slower to dry. Shellac- and water-based primers dry very quickly and under good drying conditions can be painted within a few hours with either acrylic or alkyd paint. Remember that you can have primer tinted to cut down on the finish coats, although you should aim for two coats of your finish paint for the best looks and durability.
Stain and Varnish
Some fiberglass doors can be stained with gel stain, then protected with clear varnish. Done well, such treatment can make a fiberglass door look like natural wood. Gel stain is applied with a brush or rag, and creative types can use wood-graining tools to create an even more authentic look. Once the stain is dry, apply at least three coats of clear protective finish. While it's a bit more time-consuming than painting, the results are worth it--and you can always paint over it later if you're not thrilled with the effect.