A Guide to Sewing Water Repellent

Coated Fabrics can be water repellent or waterproof.
Water Repellent fabrics have a special weave or finish on the GOOD side of the fabric that encourages water to bead off the fabric. They will take longer to absorb water, but ultimately will not keep you dry in extremely wet conditions. They are great for lined and insulated jackets and kids' clothes. Popular brand names include gemander, and Supplex.
Waterproof fabrics are coated on the WRONG side to keep water from reaching the inside. When wet, they will feel wet on the outside, but dry on the inside. These coatings can be polyurethane(PU) or polyvinylchloride (PVC). PU coatings are clear to the eye, and are thinner than PVC coatings, which tend to make the fabric stiffer and heavier. The most popular waterproof fabrics are Packcloth and Cordura. They are used for backpacks, hockey bagsand luggage.
Waterproof fabrics can also be breathable, meaning that moisture can pass OUT but not in. Popular brands are Goretex and Ultrex. They are used for sportswear such as jackets, gloves and skiwear.
Coated fabrics, including packcloth, Cordura and Goretex require special attention to retain their waterproofing.

Always make sure the coated side in on the INSIDE of your garment or bag.
If you use pins, make sure you pin WITHIN the seam allowances. Once you punch a hole in the coating, the water will gee in through that hole. Try stapling pieces together in the seam allowance, instead of using pins.
Use a polyester thread, or a nylon thread for heavier fabrics like Cordura.
Use a sharp needle, size 14 or 16, and change frequently as nylon can dull your needle quickly. A microfibre needle does not flare out towards the top and leaves smaller holes. Increase your stitch length (for fewer holes) and increase your tension slightly. As with any project, sew a test swatch first.
You can seal the seams and stitching holes with seam sealer. Seam sealers can be a liquid which is brushed on and allowed to dry; or a tape which is ironed over the holes.
Breathable fabrics have a delicate coating and should be lined to keep the coating intact.
Most coated fabrics will not fray and do not require serging, pinking or finishing. Lighter nylons and uncoated fabrics will however fray quite easily and should be finished.
PVC and PU coatings eliminate any sense of grain in fabrics. You can usually reduce the amount of fabric called for by ignoring the layout instructions and cutting pieces in the most economical way.
You can maintain your fabric by following the cleaning instructions carefully. Putting your Goretex jacket in the dryer will actually renew the waterproofing and refill any scratches or holes in the coating!