Dried gourds make sturdy, long-lasting houses for martins, as long as you're willing to put a little time into preparing the gourds. According to BirdsAndBlooms.com, gourd birdhouses can last as long as 30 years, provided you treat them with a preservative. Make sure you start with gourds that are completely dried (you should be able to hear the seeds rattling inside) and have been thoroughly cleaned before you begin constructing martin houses with them.
Mark the hole for the birdhouse opening. You want the hole to face straight out from the side of the gourd, so hold the gourd upright by its stem, letting it hang. Find the outermost curve of the gourd, which should be about one-third of the way from the bottom of the gourd, and use your permanent marker to make a small dot on the dried gourd. This is the center of your birdhouse opening. Draw a circle for the birdhouse opening that is 2 ¼ inches in diameter, with the dot you made with your marker in the center of the circle.
Cut out the birdhouse opening with a keyhole saw. Insert the keyhole saw straight into the gourd, and cut out the outlined opening. BirdsAndBlooms.com suggests that you cut the opening while your gourd is still wet from cleaning. This makes it easier for you to cut through the gourd's tough skin.
Make drainage, ventilation and hanging holes in the dried gourd. Using a 5/16-inch bit, drill six to eight holes in the bottom of the gourd for drainage. Drill four more holes at the top of the gourd, spacing them evenly and about 1 inch from the base of the gourd stem. Two of these holes are for ventilation; the other two are for hanging your martin house.
Remove the seeds and membrane from inside your gourd. Let the gourd soak in water for several hours, and then use a long-handled spoon to scoop out the seeds and the membrane. The soaking should allow these to come out easily.
Treat the gourd with a wood preservative. You should be able to find this at your local hardware store. Dip the gourd completely in the preservative for 15 to 18 minutes, and then let it hang for several days to dry completely.
Paint and hang your gourd. Use an oil-based paint for your primer, and follow it with two coats of white oil-based exterior paint. Thread thin wire through two of the holes that you made in the top of the gourd, and use the wire to hang the gourd outside about 10 to 15 feet from the ground.