Typically with these types of fabrics, the repellancy is built into the fabric membrane by the way the pores are overlapped, so it is never lost. What will get lost over time, however, is the surface repellancy, but this is more of a cosmetic issue rather than a loss of water resistance. Even though the surface may lose its ability to bead water over time, that doesn't mean it's going to leak.
The most important key to maintaining surface repellancy is to keep the fabric clean. This means the removal of dirt and oils that build up on the surface. The best way is to use a soap or detergent that is specially designed for DWR fabrics. Such soaps clean the fabric without leaving the residue that most regular detergents do, don't leave a fragrance behind, etc. Set the washer for an extra rinse cycle to ensure there is no soap residue left on the fabric, and never ever use fabric softener. In fact, if you have used fabric softener in the previous load of laundry, it's a good idea to run a load without fabric softener to ensure any residue has been cleaned out of the washer. My favorite product for washing DWR fabrics is Nikwax Tech Wash .
After washing, putting the garment in the dryer for about 10 minutes will help to reactivate the surface repellancy (check the care label first, though, because sometimes the liner might not be heat safe). This is normally all that is required to re-energize your DWR garment. If it has seen a lot of use, however, this might not be enough to restore the surface repellancy, in which case you will want to treat it with something to give the surface a helping hand (but as I said, the waterproofness is built into the fabric membrane and whether or not the surface beads water doesn't affect it, but deep penetration of oils might). Test the previous problem areas first by splashing some water on those areas to see if it beads (good) or whether it wets the surface (bad). For treating the fabric after washing, a good product that is safe for waterproof laminates is Nikwax TX Direct (Wash-In or Spray-On). The Nikwax website gives advice on which product is suitable for which fabrics, as well as how to apply it and when to use it.
Hope that helps.
UPDATE: I just looked up the main fabric that Cloudveil uses in that garment (eVent) and the eVent website has detailed information on care and maintenance. It's pretty much as I described above, and they also recommend Nikwax Tech Wash and TX Direct for washing and waterproofing. They recommend drip drying, but also say heat will help to reactivate surface repellancy (a bit of a contradiction). Ultimately they say refer to the garment manufacturers label when deciding whether to put it into the dryer; probably because some components of the garment might be susceptible to heat damage, and they have no way of knowing what the manufacturer has built into the garment.
Edited by exracer - 3/17/2009 at 05:58 am
Edited by exracer - 3/17/2009 at 05:53 am